diff --git a/crates/typst-layout/src/math/mat.rs b/crates/typst-layout/src/math/mat.rs
index 1969dda57..f1883c21d 100644
--- a/crates/typst-layout/src/math/mat.rs
+++ b/crates/typst-layout/src/math/mat.rs
@@ -87,7 +87,7 @@ pub fn layout_mat(
     let augment = elem.augment.resolve(styles);
     if let Some(aug) = &augment {
         for &offset in &aug.hline.0 {
-            if offset == 0 || offset.unsigned_abs() >= rows.len() {
+            if offset > rows.len() as isize || offset.unsigned_abs() > rows.len() + 1 {
                 bail!(
                     span,
                     "cannot draw a horizontal line after row {} of a matrix with {} rows",
@@ -98,7 +98,7 @@ pub fn layout_mat(
         }
 
         for &offset in &aug.vline.0 {
-            if offset == 0 || offset.unsigned_abs() >= ncols {
+            if offset > ncols as isize || offset.unsigned_abs() > ncols + 1 {
                 bail!(
                     span,
                     "cannot draw a vertical line after column {} of a matrix with {} columns",
@@ -165,7 +165,7 @@ fn layout_body(
         ..Default::default()
     };
 
-    let (hline, vline, stroke) = match augment {
+    let (mut hline, mut vline, stroke) = match augment {
         Some(augment) => {
             // We need to get stroke here for ownership.
             let stroke = augment.stroke.unwrap_or_default().unwrap_or(default_stroke);
@@ -216,20 +216,48 @@ fn layout_body(
         }
     }
 
+    for line in hline.0.iter_mut() {
+        if *line < 0 {
+            *line += (nrows as isize) + 1
+        }
+    }
+
+    for line in vline.0.iter_mut() {
+        if *line < 0 {
+            *line += (ncols as isize) + 1
+        }
+    }
+
     // For each row, combine maximum ascent and descent into a row height.
     // Sum the row heights, then add the total height of the gaps between rows.
-    let total_height =
+    let mut total_height =
         heights.iter().map(|&(a, b)| a + b).sum::<Abs>() + gap.y * (nrows - 1) as f64;
 
+    if hline.0.contains(&0_isize) {
+        total_height += gap.y;
+    }
+
+    if hline.0.contains(&(nrows as isize)) {
+        total_height += gap.y;
+    }
+
     // Width starts at zero because it can't be calculated until later
     let mut frame = Frame::soft(Size::new(Abs::zero(), total_height));
 
     let mut x = Abs::zero();
 
+    if vline.0.contains(&(0_isize)) {
+        frame.push(
+            Point::with_x(x + half_gap.x),
+            line_item(total_height, true, stroke.clone(), span),
+        );
+        x += gap.x;
+    }
+
     for (index, col) in cols.into_iter().enumerate() {
         let AlignmentResult { points, width: rcol } = alignments(&col);
 
-        let mut y = Abs::zero();
+        let mut y = if hline.0.contains(&0_isize) { gap.y } else { Abs::zero() };
 
         for (cell, &(ascent, descent)) in col.into_iter().zip(&heights) {
             let cell = cell.into_line_frame(&points, alternator);
@@ -251,9 +279,7 @@ fn layout_body(
         x += rcol;
 
         // If a vertical line should be inserted after this column
-        if vline.0.contains(&(index as isize + 1))
-            || vline.0.contains(&(1 - ((ncols - index) as isize)))
-        {
+        if vline.0.contains(&(index as isize + 1)) {
             frame.push(
                 Point::with_x(x + half_gap.x),
                 line_item(total_height, true, stroke.clone(), span),
@@ -264,16 +290,17 @@ fn layout_body(
         x += gap.x;
     }
 
-    // Once all the columns are laid out, the total width can be calculated
-    let total_width = x - gap.x;
+    let total_width = if !(vline.0.contains(&(ncols as isize))) { x - gap.x } else { x };
 
     // This allows the horizontal lines to be laid out
     for line in hline.0 {
-        let real_line =
-            if line < 0 { nrows - line.unsigned_abs() } else { line as usize };
-        let offset = (heights[0..real_line].iter().map(|&(a, b)| a + b).sum::<Abs>()
-            + gap.y * (real_line - 1) as f64)
-            + half_gap.y;
+        let offset = if line == 0 {
+            gap.y
+        } else {
+            (heights[0..line as usize].iter().map(|&(a, b)| a + b).sum::<Abs>()
+                + gap.y * (line - 1) as f64)
+                + half_gap.y
+        };
 
         frame.push(
             Point::with_y(offset),
diff --git a/tests/ref/math-mat-augment.png b/tests/ref/math-mat-augment.png
index 306c4b199..cede21008 100644
Binary files a/tests/ref/math-mat-augment.png and b/tests/ref/math-mat-augment.png differ
diff --git a/tests/suite/math/mat.typ b/tests/suite/math/mat.typ
index 80f190605..0e14db8f5 100644
--- a/tests/suite/math/mat.typ
+++ b/tests/suite/math/mat.typ
@@ -96,12 +96,18 @@ $ mat(column-gap: #1em, 1, 2; 3, 4)
 
   $ mat(10, 2, 3, 4; 5, 6, 7, 8; augment: #3) $,
   $ mat(10, 2, 3, 4; 5, 6, 7, 8; augment: #(-1)) $,
+  $ mat(100, 2, 3; 4, 5, 6; 7, 8, 9; augment: #(hline: 0)) $,
+  $ mat(100, 2, 3; 4, 5, 6; 7, 8, 9; augment: #(hline: -4)) $,
   $ mat(100, 2, 3; 4, 5, 6; 7, 8, 9; augment: #(hline: 2)) $,
   $ mat(100, 2, 3; 4, 5, 6; 7, 8, 9; augment: #(hline: -1)) $,
   $ mat(100, 2, 3; 4, 5, 6; 7, 8, 9; augment: #(hline: 1, vline: 1)) $,
   $ mat(100, 2, 3; 4, 5, 6; 7, 8, 9; augment: #(hline: -2, vline: -2)) $,
   $ mat(100, 2, 3; 4, 5, 6; 7, 8, 9; augment: #(vline: 2, stroke: 1pt + blue)) $,
   $ mat(100, 2, 3; 4, 5, 6; 7, 8, 9; augment: #(vline: -1, stroke: 1pt + blue)) $,
+  $ mat(10, 2, 3, 4; 5, 6, 7, 8; augment: #4) $,
+  $ mat(100, 2, 3; 4, 5, 6; 7, 8, 9; augment: #(vline: 3, stroke: 1pt + blue)) $,
+  $ mat(100, 2, 3; 4, 5, 6; 7, 8, 9; augment: #(vline: 0, stroke: 1pt + blue)) $,
+  $ mat(10, 2, 3, 4; 5, 6, 7, 8; augment: #(vline: -5, stroke: 1pt + blue)) $,
 )
 
 --- math-mat-augment-set ---
@@ -115,8 +121,8 @@ $ mat(1, 0, 0, 0; 0, 1, 0, 0; 0, 0, 1, 1) $
 #set math.mat(augment: none)
 
 --- math-mat-augment-line-out-of-bounds ---
-// Error: 3-37 cannot draw a vertical line after column 3 of a matrix with 3 columns
-$ mat(1, 0, 0; 0, 1, 1; augment: #3) $,
+// Error: 3-37 cannot draw a vertical line after column 4 of a matrix with 3 columns
+$ mat(1, 0, 0; 0, 1, 1; augment: #4) $,
 
 --- math-mat-align ---
 $ mat(-1, 1, 1; 1, -1, 1; 1, 1, -1; align: #left) $