diff --git a/btree/btree.go b/btree/btree.go
index 727af40..fc0b4d9 100644
--- a/btree/btree.go
+++ b/btree/btree.go
@@ -1,5 +1,7 @@
 package btree
 
+import "slices"
+
 type BTree[K, V any] struct {
 	cmp func(K, K) int
 
@@ -10,6 +12,7 @@ type BTree[K, V any] struct {
 // the root must have between t-1 and 2t-1 keys (inclusive).
 // Nodes with 2t-1 keys are considered "full".
 const tee = 10
+const teeFull = tee*2 - 1
 
 type node[K, V any] struct {
 	// Key ordering invariants (defining n=len(keys)):
@@ -72,3 +75,46 @@ func (bt *BTree[K, V]) getFromNode(key K, n *node[K, V]) (v V, ok bool) {
 	// recurse into children[i]
 	return bt.getFromNode(key, n.children[i])
 }
+
+// splitChild splits n.children[i] into two children nodes and moves the
+// median key of n.children[i] into n. It assumes that n isn't full,
+// but n.children[i] is full.
+func (bt *BTree[K, V]) splitChild(n *node[K, V], i int) {
+	// Notation: y is the i-th child of n (the one being split), and z is the
+	// new node created to adopt y's t-1 largest keys.
+	y := n.children[i]
+	if len(n.keys) == teeFull || len(y.keys) != teeFull {
+		panic("expect n to be non-full and y to be full")
+	}
+	z := &node[K, V]{
+		leaf: y.leaf,
+	}
+
+	// Move keys to z.
+	// Before the move, y.keys:
+	//
+	//   k[0]  k[1] ... k[t-2]  k[t-1]  k[t]  k[t+1] ... k[2t-2]
+	//
+	// k[t-1] is the median key -- it will move to n.
+	// k[0]..k[t-2] will stay with y
+	// k[t]..k[2t-2] will move to z
+	z.keys = make([]nodeKey[K, V], tee-1)
+	medianKey := y.keys[tee-1]
+	copy(z.keys, y.keys[tee:])
+	y.keys = y.keys[:tee-1]
+
+	// Move children to z.
+	//
+	// The first t children stay with y; the other t children move to z.
+	if !y.leaf {
+		z.children = make([]*node[K, V], tee)
+		copy(z.children, y.children[tee:])
+		y.children = y.children[:tee]
+	}
+
+	// Place the median key in n
+	n.keys = slices.Insert(n.keys, i, medianKey)
+
+	// Place the pointer to z after the pointer to y in n
+	n.children = slices.Insert(n.children, i+1, z)
+}