Stonehenge - Central Horseshoe 

At the center of Stonehenge is a horseshoe that originally consisted of five trilithons (see the photo below) one of which was larger than the other and was the center of the horseshoe. It is called the Great Trilithon.  Only three of the smaller trilithons and one of the vertical stones of the Great Trilion now remain.

A trilithon is a structure that looks like a gate, built with two vertical stones and a horizontal stone called a lintel.   A lintel is secured to the vertical stones by mortise and tenan joints.