diff --git a/core/include/vf/Selection.h b/core/include/vf/Selection.h
new file mode 100644
index 0000000..d4ae001
--- /dev/null
+++ b/core/include/vf/Selection.h
@@ -0,0 +1,153 @@
+#pragma once
+
+#include <cstddef>
+#include <cstdint>
+#include <functional>
+#include <vector>
+
+#include "Block.h"
+#include "Math.h"
+#include "World.h"
+
+namespace vf {
+
+// How a selection was built. Every shape collapses into a SelectionMask, so the
+// tag is informational: it survives shifting but any set operation produces a
+// free-form Mask, because the result no longer has a closed form.
+enum class SelectionShape : uint8_t {
+    Box = 0,
+    Sphere = 1,
+    Ellipsoid = 2,
+    Cylinder = 3,
+    HollowBox = 4,
+    Wand = 5,
+    Mask = 6,
+};
+
+// One bit per cell of an axis-aligned bounding box, indexed in the same
+// (y, z, x) order the chunk storage uses. Storing the box explicitly is what
+// makes union/difference/intersection and in-box negation exact rather than
+// approximations over a list of positions.
+class SelectionMask {
+public:
+    SelectionMask() = default;
+    SelectionMask(const Vec3i& minCorner, const Vec3i& dims);
+
+    const Vec3i& min() const { return corner; }
+    Vec3i max() const;
+    const Vec3i& dims() const { return size; }
+
+    std::size_t cellCount() const;
+    std::size_t count() const { return setCells; }
+    bool empty() const { return setCells == 0u; }
+
+    bool inBounds(const Vec3i& position) const;
+    bool contains(const Vec3i& position) const;
+
+    void add(const Vec3i& position);
+    void remove(const Vec3i& position);
+    void setCell(const Vec3i& position, bool on);
+    void fillAll();
+    void clearAll();
+
+    SelectionMask unionWith(const SelectionMask& other) const;
+    SelectionMask intersectWith(const SelectionMask& other) const;
+    SelectionMask differenceWith(const SelectionMask& other) const;
+    SelectionMask inverted() const;
+
+    // Dilation and erosion over the 6-connected neighbourhood, applied `radius`
+    // times. Dilation grows the bounding box; erosion keeps it and treats
+    // everything outside as unset, so eroding a solid box peels its shell.
+    SelectionMask dilated(int radius) const;
+    SelectionMask eroded(int radius) const;
+
+    // Cells that have at least one 6-neighbour outside the selection.
+    SelectionMask outline() const;
+
+    // Same cells, bounding box shrunk to fit them.
+    SelectionMask trimmed() const;
+    SelectionMask shifted(const Vec3i& delta) const;
+
+    bool sameCells(const SelectionMask& other) const;
+
+    void forEach(const std::function<void(const Vec3i&)>& visit) const;
+    std::vector<Vec3i> cells() const;
+
+private:
+    std::size_t indexOf(const Vec3i& position) const;
+    void recount();
+
+    Vec3i corner{0, 0, 0};
+    Vec3i size{0, 0, 0};
+    std::vector<uint64_t> words;
+    std::size_t setCells = 0;
+};
+
+// Aggregate view of what a selection covers in a given world.
+struct SelectionStats {
+    std::size_t blockCount = 0;
+    std::size_t solidCount = 0;
+    std::size_t airCount = 0;
+    Vec3i dims{0, 0, 0};
+    long long boundingVolume = 0;
+    std::vector<uint32_t> histogram;
+
+    uint32_t countOf(Block block) const;
+    Block dominantBlock() const;
+    std::size_t distinctBlocks() const;
+};
+
+// A world-space selection: a shape tag plus the mask it rasterized into.
+// Every operation returns a new selection; nothing mutates in place, so a
+// selection handed to a brush cannot change under it.
+class Selection {
+public:
+    Selection() = default;
+
+    static Selection box(const Vec3i& cornerA, const Vec3i& cornerB);
+    static Selection sphere(const Vec3i& center, int radius);
+    static Selection ellipsoid(const Vec3i& center, const Vec3i& radii);
+    static Selection cylinder(const Vec3i& center, int radius, int height);
+    static Selection hollowBox(const Vec3i& cornerA, const Vec3i& cornerB, int thickness);
+    static Selection fromMask(const SelectionMask& mask);
+
+    // Flood fill over blocks equal to the one at `seed`, 6-connected, stopping
+    // at `maxCells`. The limit is what keeps a wand on an ocean of air finite,
+    // so hitting it is a normal outcome reported by truncated().
+    static Selection magicWand(const World& world, const Vec3i& seed, std::size_t maxCells);
+    static Selection magicWandWithin(const World& world, const Vec3i& seed, std::size_t maxCells,
+                                     const Vec3i& limitMin, const Vec3i& limitMax);
+
+    SelectionShape shape() const { return kind; }
+    const SelectionMask& mask() const { return cells; }
+    bool truncated() const { return hitLimit; }
+
+    bool contains(const Vec3i& position) const { return cells.contains(position); }
+    std::size_t count() const { return cells.count(); }
+    bool empty() const { return cells.empty(); }
+    Vec3i min() const { return cells.min(); }
+    Vec3i max() const { return cells.max(); }
+    Vec3i dims() const { return cells.dims(); }
+
+    Selection unionWith(const Selection& other) const;
+    Selection intersectWith(const Selection& other) const;
+    Selection differenceWith(const Selection& other) const;
+    Selection inverted() const;
+    Selection expanded(int radius) const;
+    Selection contracted(int radius) const;
+    Selection outline() const;
+    Selection shifted(const Vec3i& delta) const;
+
+    SelectionStats stats(const World& world) const;
+    std::vector<Vec3i> blocks() const;
+    void forEach(const std::function<void(const Vec3i&)>& visit) const;
+
+private:
+    static Selection derived(SelectionMask mask);
+
+    SelectionShape kind = SelectionShape::Mask;
+    SelectionMask cells;
+    bool hitLimit = false;
+};
+
+} // namespace vf