Abstract: A sidebar is provided for a mechanical lock. The lock includes a cylinder rotatable within a sleeve when a mated key is inserted in the cylinder and the key and the cylinder are rotated within the sleeve. The sidebar prevents rotation of the key and the cylinder when the mated key is not present in the cylinder. The sidebar includes a base portion and a body portion extending therefrom. The body portion has first and second opposed surfaces and at least one end. A pair of diverging chamfer surfaces extend from the end of the body portion of the sidebar. Each diverging chamfer surface intersects a distinct opposed surface on the body portion. The chamfer surfaces causes an increase in the shear strength of the sidebar so as to make forced rotation of the cylinder more difficult.