Rules are a crude measuring instrument when compared to micrometers. There are many different styles and sizes of rules. This information will pertain to the common flat rule. Also called a "scale". A rule or scale is divided into graduations. The length or spacing between the graduations can start at 1/64 of inch or 1 centimeter and increase depending on the purpose of the rule.
A rule should indicate the size of the smallest graduation on it's scale.
If the scale of the graduations are unknown, just count the number of graduations between one inch. The rule below is divided into 16ths of an inch.
These graduations come in different lengths to denote size. The smallest would be a 1/16" graduation. The second smallest would be 1/8" (1/16 + 1/16= 2/16). The third is 1/4" (4 x 1/164/16). The fourth (6) would be 3/8" (6 x 1/16= 6/16). The second tallest is 1/2" (8 x 1/16= 8/16). The tallest(16) usually has a number next to it indicating 1 inch. When ever possible you should always start your measurement from another graduation other than 0. Since the end of the rule usually gets bent or worn.
By counting the largest graduations first. The screw is at least 2 inches long. Next count the remaining graduations (4/16). The screw above measures 2 and 4/16 of inch long. (2 1/4") When measuring a bolt. You need to know the features of a bolt.
The grip, shank, and thread lengths are different measurements. Just start at a point and count the graduations. Whether it's one foot, two inches or just a sixteenth of an inch it will just add up. It's that simple!