The "lap" and "gauge" are generally more accurately defined as follows: -
The "lap" is the distance between the tail of any course and the nail hole of the next course but one under it.
The "gauge" is half the difference between the length of the slate (measuring from the nail hole) and the lap.
For example, with "ladies" (16"x 8") nailed at the head, as shown in Fig. 452, 16 in.-1in.*-3in./2=12in./2= 6 inches (the gauge); with "countess" slates (20" x 10" ) similarly nailed, 20 in. -1in.* - 3in./2=16in./2=8 inches (the gauge).
With countess slates nailed at head having a 4" lap the gauge would be 20in. 1in.*-4in./2=7 1/2 inches.
* One inch is deducted from the full length of the slate, being the distance from the nail hole to the head.
The last-mentioned definition of lap and gauge refers, however, only to slates nailed near the head (see Fig. 451). When slates are nailed near the centre (see Fig. 453) the lap is the distance between the tail of any course and the head of the course next but one below, and the gauge is equal to half the difference between the lap and the full length of the slates.
Thus with countess slates, 20 inches by 10 inches, nailed near the centre, the lap being 3 inches, the gauge is 20in. -3in./2=17in./2=8 1/2 inches.
For countess slates nailed near the centre and laid with 4 inch lap the gauge would be 20in. -4in./2 = 8 inches.
The gauges for all the different sizes of slates nailed in either way are given at p. 213.