Part 18. Introduction to decliners.

A vertical sundial on a truly south-facing East/West wall has a central
vertical noon line and a symmetrical arrangement of hour lines.
The hours will begin at 6am and end at 6pm

Sundials on walls lying at an angle to truly East/West are said to ‘decline’.
In most cases the noon line will still be vertical but the gnomon
will be rotated to so that it continues to align with the Earth’s axis.