This demonstration model shows the relationship between an Equatorial Dial (the blue/yellow disc) and Vertical, Polar and Horizontal sundials.  The red gnomon or 'polestyle' casts a simultaneous shadow indicating the same time on all four dials.  Perhaps this demonstrates why the equatorial dial is often referred to as the 'master' sundial. All these dials are in front of a south-facing wall but on the other side there is a north-facing sundial. What do you think it looks like?  It is only illuminated by the sun for a few hours each day in the summer and never after 6.0 am or before 6.0 pm.