saddle tenon

bridle joint

bridle joint, 1
In carpentry:
1. A joint formed by two timbers, of which one is hollowed out to receive the end of the other (with recessed sides).
2. A joint in which two tongues project from the sides of the tenoned member; these tongues fit into corresponding slots in the mortised member.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.