diff --git a/cruciverb-core/src/grid/mod.rs b/cruciverb-core/src/grid/mod.rs
new file mode 100644
index 0000000..2cc9b04
--- /dev/null
+++ b/cruciverb-core/src/grid/mod.rs
@@ -0,0 +1,114 @@
+pub mod pattern;
+pub mod slot;
+pub mod symmetry;
+
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+pub enum Cell {
+    Black,
+    Empty,
+    Letter(char),
+}
+
+impl Cell {
+    pub fn is_black(&self) -> bool {
+        matches!(self, Cell::Black)
+    }
+
+    pub fn is_white(&self) -> bool {
+        !self.is_black()
+    }
+
+    pub fn letter(&self) -> Option<char> {
+        match self {
+            Cell::Letter(c) => Some(*c),
+            _ => None,
+        }
+    }
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Grid {
+    pub width: usize,
+    pub height: usize,
+    cells: Vec<Cell>,
+}
+
+impl Grid {
+    pub fn new(width: usize, height: usize) -> Self {
+        Self {
+            width,
+            height,
+            cells: vec![Cell::Empty; width * height],
+        }
+    }
+
+    pub fn new_black(width: usize, height: usize) -> Self {
+        Self {
+            width,
+            height,
+            cells: vec![Cell::Black; width * height],
+        }
+    }
+
+    pub fn get(&self, row: usize, col: usize) -> Cell {
+        self.cells[row * self.width + col]
+    }
+
+    pub fn set(&mut self, row: usize, col: usize, cell: Cell) {
+        self.cells[row * self.width + col] = cell;
+    }
+
+    pub fn in_bounds(&self, row: usize, col: usize) -> bool {
+        row < self.height && col < self.width
+    }
+
+    pub fn black_count(&self) -> usize {
+        self.cells.iter().filter(|c| c.is_black()).count()
+    }
+
+    pub fn white_count(&self) -> usize {
+        self.cells.iter().filter(|c| c.is_white()).count()
+    }
+
+    pub fn density(&self) -> f64 {
+        self.black_count() as f64 / self.cells.len() as f64
+    }
+
+    pub fn total_cells(&self) -> usize {
+        self.cells.len()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn new_grid_is_all_empty() {
+        let g = Grid::new(5, 5);
+        for r in 0..5 {
+            for c in 0..5 {
+                assert_eq!(g.get(r, c), Cell::Empty);
+            }
+        }
+    }
+
+    #[test]
+    fn set_and_get() {
+        let mut g = Grid::new(3, 3);
+        g.set(1, 2, Cell::Black);
+        g.set(0, 0, Cell::Letter('A'));
+        assert_eq!(g.get(1, 2), Cell::Black);
+        assert_eq!(g.get(0, 0), Cell::Letter('A'));
+        assert_eq!(g.get(0, 1), Cell::Empty);
+    }
+
+    #[test]
+    fn density_calculation() {
+        let mut g = Grid::new(2, 2);
+        g.set(0, 0, Cell::Black);
+        assert!((g.density() - 0.25).abs() < f64::EPSILON);
+    }
+}
diff --git a/cruciverb-core/src/grid/slot.rs b/cruciverb-core/src/grid/slot.rs
new file mode 100644
index 0000000..bf10fe1
--- /dev/null
+++ b/cruciverb-core/src/grid/slot.rs
@@ -0,0 +1,186 @@
+use serde::{Deserialize, Serialize};
+
+use super::Grid;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum Direction {
+    Across,
+    Down,
+}
+
+#[derive(Debug, Clone)]
+pub struct Intersection {
+    pub other_slot: usize,
+    pub self_pos: usize,
+    pub other_pos: usize,
+}
+
+#[derive(Debug, Clone)]
+pub struct Slot {
+    pub index: usize,
+    pub row: usize,
+    pub col: usize,
+    pub direction: Direction,
+    pub length: usize,
+    pub intersections: Vec<Intersection>,
+}
+
+impl Slot {
+    pub fn cells(&self) -> Vec<(usize, usize)> {
+        (0..self.length)
+            .map(|i| match self.direction {
+                Direction::Across => (self.row, self.col + i),
+                Direction::Down => (self.row + i, self.col),
+            })
+            .collect()
+    }
+}
+
+pub fn extract_slots(grid: &Grid, min_length: usize) -> Vec<Slot> {
+    let mut slots = Vec::new();
+
+    // across slots
+    for r in 0..grid.height {
+        let mut c = 0;
+        while c < grid.width {
+            if grid.get(r, c).is_white() {
+                let start = c;
+                while c < grid.width && grid.get(r, c).is_white() {
+                    c += 1;
+                }
+                let len = c - start;
+                if len >= min_length {
+                    slots.push(Slot {
+                        index: slots.len(),
+                        row: r,
+                        col: start,
+                        direction: Direction::Across,
+                        length: len,
+                        intersections: Vec::new(),
+                    });
+                }
+            } else {
+                c += 1;
+            }
+        }
+    }
+
+    // down slots
+    for c in 0..grid.width {
+        let mut r = 0;
+        while r < grid.height {
+            if grid.get(r, c).is_white() {
+                let start = r;
+                while r < grid.height && grid.get(r, c).is_white() {
+                    r += 1;
+                }
+                let len = r - start;
+                if len >= min_length {
+                    slots.push(Slot {
+                        index: slots.len(),
+                        row: start,
+                        col: c,
+                        direction: Direction::Down,
+                        length: len,
+                        intersections: Vec::new(),
+                    });
+                }
+            } else {
+                r += 1;
+            }
+        }
+    }
+
+    compute_intersections(&mut slots);
+    slots
+}
+
+fn compute_intersections(slots: &mut Vec<Slot>) {
+    let n = slots.len();
+    // build a map of (row, col) -> (slot_index, position_in_slot)
+    let mut cell_map: std::collections::HashMap<(usize, usize), Vec<(usize, usize)>> =
+        std::collections::HashMap::new();
+
+    for slot in slots.iter() {
+        for (pos, (r, c)) in slot.cells().iter().enumerate() {
+            cell_map.entry((*r, *c)).or_default().push((slot.index, pos));
+        }
+    }
+
+    let mut intersections: Vec<Vec<Intersection>> = vec![Vec::new(); n];
+
+    for entries in cell_map.values() {
+        if entries.len() == 2 {
+            let (s1, p1) = entries[0];
+            let (s2, p2) = entries[1];
+            intersections[s1].push(Intersection {
+                other_slot: s2,
+                self_pos: p1,
+                other_pos: p2,
+            });
+            intersections[s2].push(Intersection {
+                other_slot: s1,
+                self_pos: p2,
+                other_pos: p1,
+            });
+        }
+    }
+
+    for (i, slot) in slots.iter_mut().enumerate() {
+        slot.intersections = std::mem::take(&mut intersections[i]);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::grid::Cell;
+
+    fn make_mini_grid() -> Grid {
+        // 3x3 grid with center black:
+        // . . .
+        // . # .
+        // . . .
+        let mut g = Grid::new(3, 3);
+        g.set(1, 1, Cell::Black);
+        g
+    }
+
+    #[test]
+    fn extract_across_slots() {
+        let g = make_mini_grid();
+        let slots = extract_slots(&g, 2);
+        let across: Vec<_> = slots
+            .iter()
+            .filter(|s| s.direction == Direction::Across)
+            .collect();
+        assert_eq!(across.len(), 2); // top row (3) and no middle across (1+1 too short)
+        assert!(across.iter().any(|s| s.row == 0 && s.length == 3));
+        assert!(across.iter().any(|s| s.row == 2 && s.length == 3));
+    }
+
+    #[test]
+    fn extract_down_slots() {
+        let g = make_mini_grid();
+        let slots = extract_slots(&g, 2);
+        let down: Vec<_> = slots
+            .iter()
+            .filter(|s| s.direction == Direction::Down)
+            .collect();
+        assert_eq!(down.len(), 2); // left col (3) and right col (3)
+        assert!(down.iter().any(|s| s.col == 0 && s.length == 3));
+        assert!(down.iter().any(|s| s.col == 2 && s.length == 3));
+    }
+
+    #[test]
+    fn intersections_found() {
+        let g = Grid::new(3, 3); // fully white
+        let slots = extract_slots(&g, 3);
+        // 3 across + 3 down = 6 slots, but only rows/cols of length 3
+        // actually: 3 across (one per row) + 3 down (one per col)
+        for slot in &slots {
+            assert!(!slot.intersections.is_empty());
+        }
+    }
+}