diff --git a/src/edit_mode/helix/mod.rs b/src/edit_mode/helix/mod.rs
index 4b5eac18..2ed4091d 100644
--- a/src/edit_mode/helix/mod.rs
+++ b/src/edit_mode/helix/mod.rs
@@ -3,6 +3,7 @@ mod bindings;
 mod event;
 mod key;
 mod mode;
+mod range;
 
 use crate::{
     edit_mode::EditMode,
diff --git a/src/edit_mode/helix/range.rs b/src/edit_mode/helix/range.rs
new file mode 100644
index 00000000..6ad82c46
--- /dev/null
+++ b/src/edit_mode/helix/range.rs
@@ -0,0 +1,334 @@
+#![allow(dead_code)]
+
+/// The direction a range extends in.
+///
+/// `Forward` when `head >= anchor`, `Backward` when `head < anchor`.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub(super) enum Direction {
+    Forward,
+    Backward,
+}
+
+/// A selection range.
+///
+/// Uses gap indexing — `anchor` and `head` represent positions *between* bytes,
+/// not bytes themselves. Ranges are inclusive on the left and exclusive on the
+/// right, regardless of anchor/head ordering.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub(super) struct HelixRange {
+    /// The anchor of the range: the side that doesn't move when extending.
+    anchor: usize,
+    /// The head of the range, moved when extending.
+    head: usize,
+}
+
+impl HelixRange {
+    pub(super) fn new(anchor: usize, head: usize) -> Self {
+        Self { anchor, head }
+    }
+
+    /// A zero-width range at `head`.
+    pub(super) fn point(head: usize) -> Self {
+        Self::new(head, head)
+    }
+
+    /// Start of the range
+    pub(super) fn start(&self) -> usize {
+        self.anchor.min(self.head)
+    }
+
+    /// End of the range
+    pub(super) fn end(&self) -> usize {
+        self.anchor.max(self.head)
+    }
+
+    /// Total length of the range.
+    pub(super) fn len(&self) -> usize {
+        self.end() - self.start()
+    }
+
+    /// `true` when anchor and head are at the same position.
+    pub(super) fn is_empty(&self) -> bool {
+        self.anchor == self.head
+    }
+
+    /// `Forward` when `head >= anchor`, `Backward` otherwise.
+    pub(super) fn direction(&self) -> Direction {
+        if self.head < self.anchor {
+            Direction::Backward
+        } else {
+            Direction::Forward
+        }
+    }
+
+    /// Swap anchor and head.
+    pub(super) fn flip(self) -> Self {
+        Self {
+            anchor: self.head,
+            head: self.anchor,
+        }
+    }
+
+    /// Return the range if it already points in `direction`, otherwise flip it.
+    pub(super) fn with_direction(self, direction: Direction) -> Self {
+        if self.direction() == direction {
+            self
+        } else {
+            self.flip()
+        }
+    }
+
+    /// Grow the range to cover at least `[from, to]`, preserving anchor/head
+    /// ordering.
+    ///
+    /// If the range is currently `Forward`, the anchor can only move left and
+    /// the head can only move right. If `Backward`, the roles are inverted.
+    pub(super) fn extend(self, from: usize, to: usize) -> Self {
+        debug_assert!(from <= to);
+        if self.anchor <= self.head {
+            Self {
+                anchor: self.anchor.min(from),
+                head: self.head.max(to),
+            }
+        } else {
+            Self {
+                anchor: self.anchor.max(to),
+                head: self.head.min(from),
+            }
+        }
+    }
+
+    /// `true` if `pos` lies inside the range (left-inclusive, right-exclusive).
+    pub(super) fn contains(&self, pos: usize) -> bool {
+        self.start() <= pos && pos < self.end()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn contains() {
+        let range = HelixRange::new(10, 12);
+
+        assert!(!range.contains(9));
+        assert!(range.contains(10));
+        assert!(range.contains(11));
+        assert!(!range.contains(12));
+        assert!(!range.contains(13));
+
+        let range = HelixRange::new(9, 6);
+        assert!(!range.contains(9));
+        assert!(range.contains(7));
+        assert!(range.contains(6));
+    }
+
+    #[test]
+    fn point_constructs_empty_range_at_head() {
+        let range = HelixRange::point(5);
+        assert_eq!(range.start(), 5);
+        assert_eq!(range.end(), 5);
+        assert!(range.is_empty());
+    }
+
+    #[test]
+    fn new_preserves_anchor_and_head_order() {
+        let forward = HelixRange::new(2, 5);
+        assert_eq!(forward.direction(), Direction::Forward);
+
+        let backward = HelixRange::new(5, 2);
+        assert_eq!(backward.direction(), Direction::Backward);
+    }
+
+    #[test]
+    fn start_returns_lower_of_anchor_and_head() {
+        assert_eq!(HelixRange::new(2, 5).start(), 2);
+        assert_eq!(HelixRange::new(5, 2).start(), 2);
+    }
+
+    #[test]
+    fn end_returns_higher_of_anchor_and_head() {
+        assert_eq!(HelixRange::new(2, 5).end(), 5);
+        assert_eq!(HelixRange::new(5, 2).end(), 5);
+    }
+
+    #[test]
+    fn start_and_end_agree_for_empty_range() {
+        let range = HelixRange::point(7);
+        assert_eq!(range.start(), range.end());
+    }
+
+    #[test]
+    fn len_is_zero_for_empty_range() {
+        assert_eq!(HelixRange::point(7).len(), 0);
+    }
+
+    #[test]
+    fn len_ignores_direction() {
+        assert_eq!(HelixRange::new(2, 5).len(), 3);
+        assert_eq!(HelixRange::new(5, 2).len(), 3);
+    }
+
+    #[test]
+    fn is_empty_true_when_anchor_equals_head() {
+        assert!(HelixRange::new(5, 5).is_empty());
+        assert!(HelixRange::point(0).is_empty());
+    }
+
+    #[test]
+    fn is_empty_false_for_nonzero_width() {
+        assert!(!HelixRange::new(2, 5).is_empty());
+        assert!(!HelixRange::new(5, 2).is_empty());
+    }
+
+    #[test]
+    fn direction_forward_when_head_greater_than_anchor() {
+        assert_eq!(HelixRange::new(2, 5).direction(), Direction::Forward);
+    }
+
+    #[test]
+    fn direction_backward_when_head_less_than_anchor() {
+        assert_eq!(HelixRange::new(5, 2).direction(), Direction::Backward);
+    }
+
+    #[test]
+    fn direction_forward_for_empty_range() {
+        assert_eq!(HelixRange::point(5).direction(), Direction::Forward);
+    }
+
+    #[test]
+    fn flip_swaps_anchor_and_head() {
+        let flipped = HelixRange::new(2, 5).flip();
+        assert_eq!(flipped, HelixRange::new(5, 2));
+    }
+
+    #[test]
+    fn flip_twice_returns_original() {
+        let range = HelixRange::new(2, 5);
+        assert_eq!(range.flip().flip(), range);
+    }
+
+    #[test]
+    fn flip_of_empty_range_is_unchanged() {
+        let range = HelixRange::point(5);
+        assert_eq!(range.flip(), range);
+    }
+
+    #[test]
+    fn with_direction_noop_when_already_forward() {
+        let range = HelixRange::new(2, 5);
+        assert_eq!(range.with_direction(Direction::Forward), range);
+    }
+
+    #[test]
+    fn with_direction_noop_when_already_backward() {
+        let range = HelixRange::new(5, 2);
+        assert_eq!(range.with_direction(Direction::Backward), range);
+    }
+
+    #[test]
+    fn with_direction_flips_forward_to_backward() {
+        let range = HelixRange::new(2, 5);
+        assert_eq!(
+            range.with_direction(Direction::Backward),
+            HelixRange::new(5, 2)
+        );
+    }
+
+    #[test]
+    fn with_direction_flips_backward_to_forward() {
+        let range = HelixRange::new(5, 2);
+        assert_eq!(
+            range.with_direction(Direction::Forward),
+            HelixRange::new(2, 5)
+        );
+    }
+
+    #[test]
+    fn with_direction_on_empty_range_stays_forward() {
+        let range = HelixRange::point(5);
+        assert_eq!(range.with_direction(Direction::Forward), range);
+        // Empty range is already Forward, so asking for Backward flips it —
+        // which is still the same point, since anchor == head.
+        assert_eq!(range.with_direction(Direction::Backward), range);
+    }
+
+    #[test]
+    fn extend_forward_shrinks_anchor_left() {
+        let range = HelixRange::new(5, 8);
+        assert_eq!(range.extend(2, 3), HelixRange::new(2, 8));
+    }
+
+    #[test]
+    fn extend_forward_grows_head_right() {
+        let range = HelixRange::new(2, 5);
+        assert_eq!(range.extend(6, 8), HelixRange::new(2, 8));
+    }
+
+    #[test]
+    fn extend_forward_grows_both_sides() {
+        let range = HelixRange::new(4, 6);
+        assert_eq!(range.extend(2, 8), HelixRange::new(2, 8));
+    }
+
+    #[test]
+    fn extend_forward_noop_when_range_already_covers() {
+        let range = HelixRange::new(1, 9);
+        assert_eq!(range.extend(3, 5), range);
+    }
+
+    #[test]
+    fn extend_backward_preserves_direction() {
+        let range = HelixRange::new(8, 2);
+        let result = range.extend(4, 6);
+        assert_eq!(result.direction(), Direction::Backward);
+    }
+
+    #[test]
+    fn extend_backward_grows_head_left() {
+        let range = HelixRange::new(8, 5);
+        assert_eq!(range.extend(2, 3), HelixRange::new(8, 2));
+    }
+
+    #[test]
+    fn extend_backward_grows_anchor_right() {
+        let range = HelixRange::new(5, 2);
+        assert_eq!(range.extend(6, 8), HelixRange::new(8, 2));
+    }
+
+    #[test]
+    fn extend_from_empty_range_stays_forward() {
+        let range = HelixRange::point(5);
+        let result = range.extend(3, 7);
+        assert_eq!(result.direction(), Direction::Forward);
+        assert_eq!(result, HelixRange::new(3, 7));
+    }
+
+    #[test]
+    fn extend_with_zero_width_target_is_safe() {
+        let range = HelixRange::new(2, 5);
+        assert_eq!(range.extend(3, 3), range);
+    }
+
+    #[test]
+    fn contains_false_for_empty_range() {
+        let range = HelixRange::point(5);
+        assert!(!range.contains(5));
+        assert!(!range.contains(4));
+        assert!(!range.contains(6));
+    }
+
+    #[test]
+    fn contains_is_direction_agnostic() {
+        let forward = HelixRange::new(2, 5);
+        let backward = HelixRange::new(5, 2);
+        for pos in 0..=6 {
+            assert_eq!(
+                forward.contains(pos),
+                backward.contains(pos),
+                "mismatch at {pos}"
+            );
+        }
+    }
+}