diff --git a/builtin/commands/default.lua b/builtin/commands/default.lua
index b220a74..eb0fb3a 100644
--- a/builtin/commands/default.lua
+++ b/builtin/commands/default.lua
@@ -219,6 +219,12 @@ cmd { name = "window.split-horizontal",
 cmd { name = "window.split-vertical",
       description = "Split the active window vertically (children sit side-by-side).",
       fn = function() pmacs.window.split_vertical() end }
+cmd { name = "window.toggle-line-numbers",
+      description = "Toggle the active window's line-number gutter (off / absolute).",
+      fn = function()
+        local cur = pmacs.window.line_numbers()
+        pmacs.window.set_line_numbers(cur == "off" and "absolute" or "off")
+      end }
 cmd { name = "window.focus-next",
       description = "Move focus to the next window in iteration order.",
       fn = function() pmacs.window.focus_next() end }
diff --git a/src/editor.rs b/src/editor.rs
index 95bd64d..41108c9 100644
--- a/src/editor.rs
+++ b/src/editor.rs
@@ -882,7 +882,20 @@ impl EditorState {
         };
         let inner_rows = rect.size.rows.saturating_sub(1);
         let local_row = cell_row.saturating_sub(rect.origin.row);
-        let local_col = cell_col.saturating_sub(rect.origin.col);
+        // UX gutter (Q#UX6): subtract the reserved gutter width so the
+        // hit-test lands on the right text byte. A click inside the gutter
+        // strip (raw < gutter_w) saturates to column 0 → the start of that
+        // line, a mild, useful affordance for the MVP.
+        let gutter_w = {
+            let core = self.core.borrow();
+            core.windows.get(&win_id).map_or(0, |w| {
+                let g = w.gutter_width();
+                if g >= rect.size.cols { 0 } else { g }
+            })
+        };
+        let local_col = cell_col
+            .saturating_sub(rect.origin.col)
+            .saturating_sub(gutter_w);
 
         match ev.kind {
             MouseEventKind::Down(MouseButton::Left) => {
@@ -1608,11 +1621,20 @@ pub fn paint_frame(
             continue;
         };
         let viewport_buffer_start = window.text_view.line_offset(window.view_top).unwrap_or(0);
+        // UX gutter (Q#UX2): reserve a left strip for line numbers and
+        // shrink+shift the text area into the remainder, so every
+        // viewport-relative painter (text, syntax, diagnostics, search)
+        // stays gutter-agnostic. A window too narrow for the gutter falls
+        // back to no gutter this frame rather than starving the text.
+        let gutter_w = {
+            let w = window.gutter_width();
+            if w >= rect.size.cols { 0 } else { w }
+        };
         let viewport = Viewport {
             buffer_start: viewport_buffer_start,
             buffer_end: buf.len(),
-            cell_origin: rect.origin,
-            cell_size: crate::cell::CellSize::new(inner_rows, rect.size.cols),
+            cell_origin: CellCoord::new(rect.origin.row, rect.origin.col + gutter_w),
+            cell_size: crate::cell::CellSize::new(inner_rows, rect.size.cols - gutter_w),
         };
         // Composition (T M2.9): base text_view paints first, then
         // each overlay in attach order. See [`crate::view::View`].
@@ -1620,7 +1642,10 @@ pub fn paint_frame(
         for overlay in &mut window.overlays {
             overlay.render(buf, viewport, grid);
         }
-        paint_local_selection(grid, buf, window, &rect, inner_rows);
+        if gutter_w > 0 {
+            paint_line_number_gutter(grid, window, &rect, inner_rows, gutter_w);
+        }
+        paint_local_selection(grid, buf, window, &rect, inner_rows, gutter_w);
         // Mode line for this window. Painted last so the line
         // itself is always visible regardless of overlay activity.
         let coord = window
@@ -1685,9 +1710,15 @@ pub fn paint_frame(
     {
         return None;
     }
+    // UX gutter: the terminal caret sits in the text area, past the
+    // reserved gutter strip (mirrors the viewport shift above).
+    let gutter_w = {
+        let w = aw.gutter_width();
+        if w >= active_rect.size.cols { 0 } else { w }
+    };
     let grid_row = active_rect.origin.row + (disp.row - aw.view_top as u32);
     let max_col = active_rect.origin.col + active_rect.size.cols.saturating_sub(1);
-    let grid_col = (active_rect.origin.col + disp.col).min(max_col);
+    let grid_col = (active_rect.origin.col + gutter_w + disp.col).min(max_col);
     Some(CellCoord::new(grid_row, grid_col))
 }
 
@@ -1745,12 +1776,69 @@ fn inner_rows(rect: &crate::window::Rect) -> u32 {
     rect.size.rows.saturating_sub(1)
 }
 
+/// Paint the left line-number gutter for `window` into the reserved strip
+/// `[rect.origin.col, rect.origin.col + gutter_w)` over the window's text
+/// rows (UX gutter arc). Numbers are 1-based, right-aligned with a single
+/// trailing pad cell; rows past end-of-buffer stay blank. Dimly styled so
+/// the gutter recedes behind the code. The caller guarantees `gutter_w >
+/// 0` and that it fits within `rect.size.cols`.
+fn paint_line_number_gutter(
+    grid: &mut crate::cell::CellGrid<'_>,
+    window: &crate::window::Window,
+    rect: &crate::window::Rect,
+    inner_rows: u32,
+    gutter_w: u32,
+) {
+    let line_count = window.text_view.line_count();
+    let style = crate::cell::Style {
+        fg: crate::cell::Color::Indexed(8),
+        ..crate::cell::Style::default()
+    };
+    // The number's rightmost digit sits at `field - 1`; the last gutter
+    // cell (`gutter_w - 1`) is a trailing pad separating it from the code.
+    let field = gutter_w.saturating_sub(1);
+    for r in 0..inner_rows {
+        let grid_row = rect.origin.row + r;
+        // Blank + style the whole strip first, so a number that shrank a
+        // digit (e.g. after a large delete) leaves no stale trailing glyph.
+        for c in 0..gutter_w {
+            let cell = grid.at(CellCoord::new(grid_row, rect.origin.col + c));
+            cell.glyph = crate::cell::Glyph::Char(' ');
+            cell.style = style;
+            cell.attachment = None;
+        }
+        let buffer_line = window.view_top + r as usize;
+        if buffer_line >= line_count {
+            continue; // past end-of-buffer: blank gutter
+        }
+        // Write the 1-based number right-aligned, rightmost digit first,
+        // alloc-free. `field >= digits(line_count)` by construction, so
+        // the leftmost digit always leaves at least a leading pad cell.
+        let mut val = buffer_line + 1;
+        let mut col = field;
+        loop {
+            col -= 1;
+            let digit = (val % 10) as u8;
+            grid.at(CellCoord::new(grid_row, rect.origin.col + col))
+                .glyph = crate::cell::Glyph::Char((b'0' + digit) as char);
+            val /= 10;
+            if val == 0 || col == 0 {
+                break;
+            }
+        }
+    }
+}
+
 fn paint_local_selection(
     grid: &mut crate::cell::CellGrid<'_>,
     buf: &crate::buffer::Buffer,
     window: &crate::window::Window,
     rect: &crate::window::Rect,
     inner_rows: u32,
+    // UX gutter: the reserved left-strip width; selection cells are the
+    // text-relative display column shifted right by this (Q#UX2). 0 when
+    // the gutter is off, so this is a no-op then.
+    gutter_w: u32,
 ) {
     let Some((sel_start, sel_end)) = window.region() else {
         return;
@@ -1758,6 +1846,7 @@ fn paint_local_selection(
     if inner_rows == 0 || rect.size.cols == 0 || sel_start >= sel_end {
         return;
     }
+    let text_cols = rect.size.cols.saturating_sub(gutter_w);
 
     let first_row = window.view_top;
     let last_row = first_row.saturating_add(inner_rows as usize);
@@ -1786,15 +1875,15 @@ fn paint_local_selection(
         }
 
         let row_offset = display_row.saturating_sub(first_row) as u32;
-        let start_col = start_coord.col.min(rect.size.cols);
-        let end_col = end_coord.col.min(rect.size.cols);
+        let start_col = start_coord.col.min(text_cols);
+        let end_col = end_coord.col.min(text_cols);
         if start_col >= end_col {
             continue;
         }
         for col in start_col..end_col {
             let cell = grid.at(CellCoord::new(
                 rect.origin.row + row_offset,
-                rect.origin.col + col,
+                rect.origin.col + gutter_w + col,
             ));
             cell.style.reverse = true;
         }
@@ -2196,6 +2285,49 @@ mod tests {
     use super::*;
     use crate::frontend::KeyEventKind;
 
+    #[test]
+    fn line_number_gutter_renders_right_aligned_digits() {
+        use crate::buffer::{Buffer, BufferId};
+        use crate::cell::{Cell, CellGrid, CellSize, Glyph};
+        use crate::text_view::TextView;
+        use crate::window::{LineNumberMode, Window, WindowId};
+
+        // 12 lines → decimal_digits(12) = 2, gutter_w = 2 + PAD(2) = 4.
+        let content = b"a\nb\nc\nd\ne\nf\ng\nh\ni\nj\nk\nl\n";
+        let bid = BufferId::next();
+        let buf = Buffer::from_bytes(bid, "test", content);
+        let view = TextView::new(&buf);
+        let mut window = Window::new(WindowId::next(), bid, view);
+        window.line_numbers = LineNumberMode::Absolute;
+        assert_eq!(window.gutter_width(), 4, "2-digit line count + 2 pad");
+
+        let (rows, cols) = (12u32, 20u32);
+        let mut storage = vec![Cell::default(); (rows * cols) as usize];
+        let mut grid = CellGrid {
+            cells: &mut storage,
+            stride: cols,
+            size: CellSize::new(rows, cols),
+        };
+        let rect = Rect::new(0, 0, rows, cols);
+        paint_line_number_gutter(&mut grid, &window, &rect, rows, 4);
+
+        let glyph = |r: u32, c: u32| storage[(r * cols + c) as usize].glyph.clone();
+        // Row 0 = line 1: "  1 " (digit right-aligned at col 2, col 3 = pad).
+        assert_eq!(glyph(0, 0), Glyph::Char(' '));
+        assert_eq!(glyph(0, 1), Glyph::Char(' '));
+        assert_eq!(glyph(0, 2), Glyph::Char('1'));
+        assert_eq!(glyph(0, 3), Glyph::Char(' '));
+        // Row 4 = line 5.
+        assert_eq!(glyph(4, 2), Glyph::Char('5'));
+        // Row 9 = line 10: two digits → col1='1', col2='0', col3 pad.
+        assert_eq!(glyph(9, 1), Glyph::Char('1'));
+        assert_eq!(glyph(9, 2), Glyph::Char('0'));
+        assert_eq!(glyph(9, 3), Glyph::Char(' '));
+        // Row 11 = line 12.
+        assert_eq!(glyph(11, 1), Glyph::Char('1'));
+        assert_eq!(glyph(11, 2), Glyph::Char('2'));
+    }
+
     fn fresh_with(content: &[u8]) -> EditorState {
         let s = EditorState::new();
         let new_id = s
diff --git a/src/lua_bindings/mod.rs b/src/lua_bindings/mod.rs
index 5a171d6..434f632 100644
--- a/src/lua_bindings/mod.rs
+++ b/src/lua_bindings/mod.rs
@@ -10093,6 +10093,44 @@ fn install_window_module(lua: &Lua, core: &SharedCore) -> mlua::Result<Table> {
         )?;
     }
 
+    {
+        // UX gutter: set the active window's line-number mode
+        // ("off" | "absolute"). Per-window (Q#UX5); a friendly toggle
+        // command wraps this in `builtin/`.
+        let cc = core.clone();
+        win.set(
+            "set_line_numbers",
+            lua.create_function(move |_, mode: String| {
+                let m = match mode.as_str() {
+                    "off" | "none" => crate::window::LineNumberMode::Off,
+                    "absolute" | "abs" | "on" => crate::window::LineNumberMode::Absolute,
+                    other => {
+                        return Err(mlua::Error::external(format!(
+                            "unknown line-number mode {other:?} (expected off|absolute)"
+                        )));
+                    }
+                };
+                cc.borrow_mut().active_window_mut().line_numbers = m;
+                Ok(())
+            })?,
+        )?;
+    }
+
+    {
+        // Read the active window's line-number mode as a string.
+        let cc = core.clone();
+        win.set(
+            "line_numbers",
+            lua.create_function(move |_, ()| {
+                let mode = match cc.borrow().active_window().line_numbers {
+                    crate::window::LineNumberMode::Off => "off",
+                    crate::window::LineNumberMode::Absolute => "absolute",
+                };
+                Ok(mode)
+            })?,
+        )?;
+    }
+
     {
         let cc = core.clone();
         win.set(
diff --git a/src/overlay_paint.rs b/src/overlay_paint.rs
index 72ed9b4..af0ee7f 100644
--- a/src/overlay_paint.rs
+++ b/src/overlay_paint.rs
@@ -135,6 +135,15 @@ pub fn paint_other_frontend_overlays(
             let Ok(buf) = reg.get(window.buffer_id) else {
                 continue;
             };
+            // UX gutter: this window may reserve a left strip for line
+            // numbers; a remote cursor/selection is a text-relative column
+            // shifted right by that width (0 when the gutter is off). This
+            // pass runs after `paint_frame`, so it learns the width here.
+            let gutter_w = {
+                let g = window.gutter_width();
+                if g >= rect.size.cols { 0 } else { g }
+            };
+            let text_cols = rect.size.cols.saturating_sub(gutter_w);
             // Source's byte position → display coords via THIS
             // recipient window's text_view (the recipient's view
             // of the buffer).
@@ -154,11 +163,11 @@ pub fn paint_other_frontend_overlays(
             // Column bounds: disp.col is the buffer column; window
             // doesn't horizontally scroll in v1.0, so cells past
             // rect.size.cols are simply off-grid for this window.
-            if disp.col >= rect.size.cols {
+            if disp.col >= text_cols {
                 continue;
             }
             let cursor_grid_row = rect.origin.row + row_in_window as u32;
-            let cursor_grid_col = rect.origin.col + disp.col;
+            let cursor_grid_col = rect.origin.col + gutter_w + disp.col;
             paint_cursor_cell(grid, cursor_grid_row, cursor_grid_col, color);
             if let Some(label_ch) = label {
                 paint_label_cell(grid, cursor_grid_row, cursor_grid_col, label_ch, color);
@@ -171,7 +180,9 @@ pub fn paint_other_frontend_overlays(
                 } else {
                     (sel.active, sel.anchor)
                 };
-                paint_selection_in_window(grid, buf, window, rect, inner_rows, lo, hi, color);
+                paint_selection_in_window(
+                    grid, buf, window, rect, inner_rows, gutter_w, lo, hi, color,
+                );
             }
         }
     }
@@ -223,6 +234,7 @@ fn paint_selection_in_window(
     window: &crate::window::Window,
     rect: Rect,
     inner_rows: u32,
+    gutter_w: u32,
     lo: crate::rope::Position,
     hi: crate::rope::Position,
     color: Color,
@@ -230,6 +242,7 @@ fn paint_selection_in_window(
     if lo >= hi {
         return;
     }
+    let text_cols = rect.size.cols.saturating_sub(gutter_w);
     // Walk byte positions from lo to hi, mapping each to a
     // display coord. Step in single-byte increments; pos_to_display
     // tolerates byte-boundary positions and returns None for
@@ -249,9 +262,9 @@ fn paint_selection_in_window(
             break;
         };
         match (disp.row as usize).checked_sub(window.view_top) {
-            Some(r) if r < inner_rows as usize && disp.col < rect.size.cols => {
+            Some(r) if r < inner_rows as usize && disp.col < text_cols => {
                 let grid_row = rect.origin.row + r as u32;
-                let grid_col = rect.origin.col + disp.col;
+                let grid_col = rect.origin.col + gutter_w + disp.col;
                 if grid_row < grid.size.rows && grid_col < grid.size.cols {
                     let cell = grid.at(CellCoord::new(grid_row, grid_col));
                     cell.style.underline = UnderlineStyle::Single;
diff --git a/src/window.rs b/src/window.rs
index a5ed06a..9988622 100644
--- a/src/window.rs
+++ b/src/window.rs
@@ -129,6 +129,36 @@ pub struct Selection {
     pub anchor: Position,
 }
 
+/// Line-number display mode for a window's left gutter (UX gutter arc).
+/// `Off` reserves no gutter at all — text starts at column 0, and every
+/// coordinate is unchanged (the default, matching the Emacs tradition).
+/// Additional modes (relative, hybrid) arrive in a later sub-arc.
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Default)]
+pub enum LineNumberMode {
+    /// No gutter; zero layout change.
+    #[default]
+    Off,
+    /// Absolute 1-based line numbers, right-aligned in the gutter.
+    Absolute,
+}
+
+/// Cells of horizontal padding the line-number gutter adds around the
+/// digit field: a leading and a trailing blank, so `gutter_w = digits +
+/// PAD` (Q#UX3). Kept as a named constant so both frontends can share the
+/// convention (Q#UX7). `u32` to match the cell-grid column type.
+pub const LINE_NUMBER_GUTTER_PAD: u32 = 2;
+
+/// Number of decimal digits in `n` (for `n >= 1`). Allocation-free.
+#[must_use]
+pub fn decimal_digits(mut n: usize) -> u32 {
+    let mut d = 1u32;
+    while n >= 10 {
+        n /= 10;
+        d += 1;
+    }
+    d
+}
+
 /// One leaf of the window tree: a buffer plus per-window state.
 pub struct Window {
     /// Unique identifier.
@@ -158,6 +188,9 @@ pub struct Window {
     /// render. Updated by the renderer; consumed by `cursor.page-down`
     /// / `cursor.page-up`. `0` until the first render lands.
     pub last_visible_rows: u32,
+    /// Line-number gutter mode for this window (UX gutter arc). `Off` by
+    /// default → no gutter, no coordinate change.
+    pub line_numbers: LineNumberMode,
 }
 
 impl Window {
@@ -175,6 +208,22 @@ impl Window {
             view_top: 0,
             goal_col: None,
             last_visible_rows: 0,
+            line_numbers: LineNumberMode::Off,
+        }
+    }
+
+    /// Width in cells this window's line-number gutter occupies, or `0`
+    /// when disabled (UX gutter arc, Q#UX3). `digits(line_count) + PAD`;
+    /// the renderer caps this against the window width and applies it as a
+    /// left offset to the text area. Every gutter coordinate-math site
+    /// reads this one function so the width stays consistent.
+    #[must_use]
+    pub fn gutter_width(&self) -> u32 {
+        match self.line_numbers {
+            LineNumberMode::Off => 0,
+            LineNumberMode::Absolute => {
+                decimal_digits(self.text_view.line_count().max(1)) + LINE_NUMBER_GUTTER_PAD
+            }
         }
     }
 
@@ -465,6 +514,18 @@ fn collapse_single_child_splits(node: &mut LayoutNode) {
 mod tests {
     use super::*;
 
+    #[test]
+    fn decimal_digits_counts_correctly() {
+        assert_eq!(decimal_digits(1), 1);
+        assert_eq!(decimal_digits(9), 1);
+        assert_eq!(decimal_digits(10), 2);
+        assert_eq!(decimal_digits(99), 2);
+        assert_eq!(decimal_digits(100), 3);
+        assert_eq!(decimal_digits(1000), 4);
+        // A 6-digit file → 6 digits + PAD gutter.
+        assert_eq!(decimal_digits(123_456), 6);
+    }
+
     fn id() -> WindowId {
         WindowId::next()
     }