LAP is the amount by which the valve in mid position overlaps the cylinder steam ports. Valves provided with long lap required greater travel to operate effectively but this allows a freer exhaust. and more sharply defined stages in the valve events.

LEAD is the amount by which the steam port is open when the piston is static at the end of its forward or backward stroke. This allows steam to enter the space between the cylinder end and piston face before movement of the piston takes place so ensuring maximum steam pressure at the start of the stroke.

Piston valves.
Piston valves have both advantages and disadvantages compared to slide valves.
They allow a greater flow of steam into the cylinder, and consume less power. However, they are harder to make, there is
sometimes difficulty in getting them to seal properly, they can be tricky to set up correctly, they may require more frequent maintenance, and as they are often used with higher steam pressures it must be remembered that as the steam pressure is
increased the temperature rises, and this makes more demands on the lubricant used.
It used to be said that slide valves wear in, and piston valves wear out.

A "rule of thumb" is that the ports to the cylinder should be between 130% and 200% of that on slide valves.  (The latter can be calculated as previous page.)

These superb drawings, taken from a 75 year old book,  illustrate the main features of  a piston valve.

Information on piston valves will be added here.

Drop valves. / Poppet valves. / Sleeve valves.

These are all more specialised, and .require more space (and time) than is available here.