Carriage Bolts are bolts with a dome head and a square neck threaded only at the end opposite the head, used mostly for fastening pieces of wood together, and inserted into pre-drilled holes. Zero Products Inc.
Countersunk Bolts are bolts with a flat head that tapers to the shank diameter so that it can be driven flush with the top of a countersunk hole. It may have a variety of drives such as a slot, Phillips, or Allen hex in the head. Zero Products Inc.
Left Hand Bolts are bolts with a reverse thread and is turned counterclockwise instead of clockwise to tighten. Used as a means to prevent bolt from coming loose in certain moving applications. Zero Products Inc.