
London dial

A London dial in the broadest sense can mean any sundial that is set for 51°30′ N, but more specifically refers to a engraved brass horizontal sundial with a distinctive design.
London dials were originally engraved by scientific instrument makers. The trade was heavily protected by the system of craft guilds. 
A horizontal dial.
A gnomon or style is set to point at the celestial north pole, the shadow of the sun is thrown onto the dial plate and will appear at the same position each day of the year, and this position can be calculated using trigonometry, or drawn using geometric construction. 
In the world of sundials some of the technical terms use an old form of language, so the angle to which the style is set is called the style height. The style height is identical to the geographical latitude, and in London this was 51 degrees 30 minutes or 51.50 degrees, which roughly corresponds with Westminster Bridge.
The gnomon has thickness, and thus two shadow throwing edges (the styles) one for the morning and one for the afternoon, there is a gap left on the dialplate the width of the gnomon.
A sundial displays the local apparent time, and watches that use mean time or average time will always be at a slight variance. This difference of time can be calculated and displayed on the dial – again using old language it is known as the equation of time.
A London dials show hour lines, some show half-hour markers, the quarter hours and some divisions representing half-quarters (7 and a half minutes). The equation of time is shown as a three rings, showing the words "WATCH SLOWER, WATCH FASTER", the months and dates and the minutes of time of variance.
Geometrical construction.
There are many methods to do this: the one published by Leybourn in 1652 is still popular. Though the one by Dom Francois Bedos de Celles in 1790 is more widely known.
Geometrical markup technique.
A horizontal dial, takes the equiangular hour lines of a equatorial dial and projects them onto an plane oblique to the style, the markup simulates this transformation.
The Dom Francois Bedos de Celles method (1760) otherwise known as the Waugh method (1973) 
"Caveat: These diagrams have not been tested for accuracy. The further north the wider the dial becomes."