diff --git a/mino/src/matrix.rs b/mino/src/matrix.rs
index 5164570..b2dc29a 100644
--- a/mino/src/matrix.rs
+++ b/mino/src/matrix.rs
@@ -178,12 +178,8 @@ where
         data[..self.rows].copy_from_slice(mat.data());
     }
 
-    /// Fills in the cell at the given (x,y) coordinate. Adds new rows to the top of the
-    /// matrix if necessary.
-    ///
-    /// Panics if the buffer space cannot fit the new rows.
-    pub fn set(&mut self, x: i16, y: i16) {
-        if y < 0 || !(0..COLUMNS).contains(&x) {
+    pub fn fill_row(&mut self, y: i16, mask: u16) {
+        if y < 0 {
             // OOB coordinates are considered already set
             return;
         }
@@ -195,7 +191,18 @@ where
                 .expect("y should be within available buffer space") = EMPTY_ROW;
             self.rows += 1;
         }
-        data[y] |= 1 << x;
+        data[y] |= mask;
+    }
+
+    /// Fills in the cell at the given (x,y) coordinate. Adds new rows to the top of the
+    /// matrix if necessary.
+    ///
+    /// Panics if the buffer space cannot fit the new rows.
+    #[inline]
+    pub fn set(&mut self, x: i16, y: i16) {
+        if (0..COLUMNS).contains(&x) {
+            self.fill_row(y, 1 << x);
+        }
     }
 
     /// Removes any rows that are completely filled, shifting rows above down. Returns a
@@ -378,10 +385,37 @@ mod test {
         assert_eq!(buf, mat);
     }
 
+    #[test]
+    fn test_fill() {
+        let mut buf: MatBuf<[u16; 5]> = MatBuf::new();
+        buf.fill_row(1, 0b110111); // a
+        let mat = mat! {
+            "aaa.aa....";
+            "..........";
+        };
+        assert_eq!(buf, mat);
+        buf.fill_row(3, 0b1000000000); // b
+        buf.fill_row(0, u16::MAX); // c
+        let mat = mat! {
+            ".........b";
+            "..........";
+            "aaa.aa....";
+            "cccccccccc";
+        };
+        assert_eq!(buf, mat);
+    }
+
     #[test]
     #[should_panic]
     fn test_set_oob() {
         let mut buf: MatBuf<[u16; 4]> = MatBuf::new();
         buf.set(0, 4);
     }
+
+    #[test]
+    #[should_panic]
+    fn test_fill_oob() {
+        let mut buf: MatBuf<[u16; 4]> = MatBuf::new();
+        buf.fill_row(4, 0b1001);
+    }
 }
diff --git a/mino/src/piece.rs b/mino/src/piece.rs
index da61dd4..6fe1d5d 100644
--- a/mino/src/piece.rs
+++ b/mino/src/piece.rs
@@ -1,6 +1,6 @@
 //! Data structures for representing pieces and shapes.
 
-use crate::matrix::Mat;
+use crate::matrix::{Mat, MatBuf};
 
 use core::ops::Range;
 
@@ -150,6 +150,23 @@ impl<'c> Cells<'c> {
         false
     }
 
+    /// Fills in all of the cells onto the given matrix. Panics if the piece is out of
+    /// bounds.
+    pub fn fill<T>(&self, mat: &mut MatBuf<T>)
+    where
+        T: AsRef<[u16]> + AsMut<[u16]>,
+    {
+        assert!(
+            self.x >= 0 && self.x + self.w <= mat.cols() && self.y >= 0,
+            "fill() cells oob"
+        );
+        for (dy, &row) in self.data().iter().enumerate() {
+            let y = self.y + dy as i16;
+            let mask = row << self.x; // should never overflow
+            mat.fill_row(y, mask);
+        }
+    }
+
     /// Translates the cells by the given amount in both directions.
     #[inline]
     pub fn translate(&self, dx: i16, dy: i16) -> Self {
@@ -330,4 +347,45 @@ pub mod test {
         assert_eq!(isects(1..=9, 2, W), [__, __, __, __, __, __, __, __, xx]);
         assert_eq!(isects(1..=9, 3, W), [__; 9]);
     }
+
+    fn filled<I>(locs: I) -> MatBuf
+    where
+        I: IntoIterator,
+        Loc: From<I::Item>,
+    {
+        let mut mat = MatBuf::new();
+        for loc in locs {
+            let piece = Piece {
+                ty: Tri,
+                loc: loc.into(),
+            };
+            piece.cells().fill(&mut mat);
+        }
+        mat
+    }
+
+    #[test]
+    fn test_fill() {
+        let act = filled([(0, 0)]);
+        let exp = mat! {
+            "x.........";
+            "xx........";
+        };
+        assert_eq!(act, exp);
+
+        let act = filled([(1, 1, Rot::N), (5, 0, Rot::W)]);
+        let exp = mat! {
+            ".n........";
+            ".nn..w....";
+            "....ww....";
+        };
+        assert_eq!(act, exp);
+
+        let act = filled([(1, 1, Rot::S), (1, 1, Rot::E)]);
+        let exp = mat! {
+            "sxe.......";
+            ".x........";
+        };
+        assert_eq!(act, exp);
+    }
 }