[…]thus the two opposite trilithons which are nearest to the north-west or grand entrance, were 19 feet in height, the two next opposite trilithons correspond in the height of 20 feet 3 inches, and the trilithon at the back of the stone of astronomic observation, or as erringly called the Altar-stone, raised its ascent to the astonishing height of 25 feet.
Source: wiktionary
The two trilithons forming the wall on each side of the cella therefore pass on the mutual neighbourhood to each other, despite the fact that they are set a slight distance apart. But owing to the oblique placing of the pair of trilithons nearest the head-end the spread is greatly reduced on that side; the head-trilithon stands at the exact spot where it ceases.
Source: wiktionary
Within the sarsen circle are the five impressive trilithons, forming a horseshoe-shaped ring of huge stones.
Source: wiktionary
[…]whereas the four-square gallows would have been more likely to suggest to his contemporary Blake, who normally used symbolism for its own sake, a resemblance to one of the four-square trilitha at Stonehenge.
Source: wiktionary