This is a 2" tall post holder which also includes a 1/2" diameter post. The post holder has a hole underneath that allows a 1/4-20 screw to fasten it to an optical breadboard. The post has a hole for a 6-32 x 3/8" hexagonal standoff to fit inside. This allows it to take a 6-32 mounting screw to fasten to the bottom of an optical mount. The post can move up and down in the hole in the 2" holder, and is pinned at the desired height by a thumb knob with an 8-32 thread. The post holder thumb know hole needs to be threaded before use.