Abstract: Two metal blocks, each having a cylindrical passage containing a rod, and a slot extending from one side to the cylindrical passage, also have a hole extending across the slots. A threaded member passes through the hole in one block and into threads contained in the other block beyond the slot. A compression head is mounted on the other end of the threaded member and is provided with a handle. When the handle is turned to advance the threaded member, the slots tend to close somewhat preventing movement of the rods in the blocks. When the threaded member is withdrawn slightly, the rods can be moved to desired positions relative to the blocks and the blocks can be rotated relative to each other.