Trilithon

A trilithon is a megalithic structure comprising a horizontal lintel supported by two uprights. Stonehenge includes several examples.
Found on http://www.probertencyclopaedia.com/browse/TT.HTM

Trilithon

A stone arch made from two upright slabs with one lying atop these. They are featured in Stonehenge.
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Trilithon

• (n.) A monument consisting of three stones; especially, such a monument forming a kind of doorway, as among the ancient Celts.
Found on http://thinkexist.com/dictionary/meaning/trilithon/

trilithon

trilith, trilithon A massive prehistoric stone structure, resembling a doorway, consisting of two vertical stones with a third supported horizontally by the other two.
Found on http://www.wordinfo.info/words/index/info/view_unit/2740/7
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