diff --git a/src/include/geom.h b/src/include/geom.h
new file mode 100644
index 0000000..942677e
--- /dev/null
+++ b/src/include/geom.h
@@ -0,0 +1,26 @@
+#ifndef GEOM_H
+#define GEOM_H
+
+// The maximum number of Tris in a Plate.
+#define NTRI 4
+
+typedef struct {
+    int x, y;
+} Pt;
+
+typedef struct {
+    Pt* a;
+    Pt* b;
+} Edge;
+
+typedef struct {
+    Pt a;
+    Pt b;
+    Pt c;
+} Tri;
+
+typedef struct {
+    Tri tris[NTRI];
+} Plate;
+
+#endif