Plumb can be found using a spirit level, plumb bob & string, square or laser level.
If you are plumb to your level line you are square or 90°.
Plumb flats of a set after they have been aligned at the top.
Calculations in Imperial measurement.
1 Yard = 3 feet = 3 ladder rungs
1 Foot = 12 inches
Inches are divided into fractional increments with a variety of denominators; 1/2, 1/4, 1/8, 1/10, 1/12, 1/16...
Denominators are the lower or bottom numbers of a fraction example; 2 is the lower or bottom number of ½
The larger the denominator, the smaller the fraction 1 inch = 2 x ½, or 4 x Ό, or 8 x 1/8, or 16 x 1/16 (1/10 & 1/12 scales are found on the Framing Square)