The Altar Stone
The stone standing at the centre of the monument is a block of blue-grey sandstone from Pembrokeshire in Wales. It was called the Altar stone only later, after it had fallen. The midsummer sunrise is thought to have struck the stone on the longest day of the year.
The main axis of Stonehenge is aligned not only onto the midsummer sunrise - the solstice or longest day of the year, but also to the midwinter sunset at the opposite direction to the Heel stone.
The Station Stones
The four Station Stones are also aligned with the midsummer sunrise and the midwinter sunset. They may also be connected with the most southerly moonrise and the most northerly moonset. Diagonal lines from the Station Stones cross the exact centre of the monument.
Y and Z holes
In about 1700BC a double circle of the Y and Z Holes was dug around the outside of the Sarsen circle, with possibly the intention of rearranging the Bluestones yet again. For reasons unknown, the work remained unfinished.