Original Position of the Altar Stone

In the recent discussions of the Altar Stone at Stonehenge variations of the equivocation that is unknown if it was originally vertical or if it was placed in its present horizontal position keep reoccurring.

Because Stone 55 fell and broke it there is assumed to be ambiguity over this. In the words of an engineering expert who has looked at the plans, the excavation records and scenarios of the collapse of the Trilithon onto it, the idea that it was a vertical stone is "knuckle-dragging, mouth-breathing stupidity that isn't worth a pitcher of warm spit". I wouldn't go so far myself just that the idea that has no logic or evidence behind it. 

It was placed as a horizontal stone. Which means that its 81 degree angle to the main Solstical Alignment is also deliberate.

As I argued in my "The Twisted Trilithon of Stonehenge" it is just one of the group of stones at the apse end of the inner horseshoe so aligned. They indicate the sunrise and sunset of the Solstical days that the main alignment indicates the other end of the day of. So the 81 Degree angle aligns to the sunrise of the Winter Solstice Sun which at the end of the day sets in line with the middle of the horseshoe. As well as the Altar stone (Stone 80), and the tallest Trilithon, there were a pair of Bluestone pillars 66 and 68 (68 has been pushed over but its original position is known from Gowland's excavation records), marked by blue circles in the plan below. There was also a pair of wooden posts to the north of the Altar Stone 3364 and 3362.

Plan based on Cleal et al

Intriguingly the layout is reminiscent of the Aberdeen recumbent stone circles -

   

Easter Aquhorthies stone circle (cropped view) by stu smith from Hampton Wick / Aberdeen, uk, CC BY-SA 2.0, via Wikimedia Commons

Dr John Hill goes even further "The Scottish Stonehenge Architect and His Aberdeenshire Stone Circles".