The various scales of rules are discussed here.

### Principal Scales

The principal scale on most rules is the one that measures linear length using a system of units. Usually fractions of the unit are easily determined as well. Two methods of indicating the fractions are used:
1. Open divided scales, in this case the distance of one unit interval is divided in the appropiate fractions and placed prior to the 0 position. A common example is the drafting scale.
2. Fully divided scales, in this case each unit interval is divided with the fractions, not necessarily the same for all units. A common example is the ordinary desk rule.

### Secondary Scales

Many other scales are used as well, these are usually to provide a reference for setting dividers or the rapid answer to a complex mathamatical problem.

### Eight Square Scale or E & M Scale

Rabone's catalog describes these scales for mast or sparmakers to use when determining the width of a face for a given square timber. These E & M Scales are used to determine the distance from the edge (using the E scale) or from the middle (using the M scale) for a line to be marked. These scales are usually found along the inner edges of a 2 foot 2 fold carpenters rule along the first 7 inches or so. Along one inner edge the M scale usually runs Right to Left from 0 to 34 and along the other inner edge the E scale usually runs Right to Left from 0 to 24. Both scales are usually fully divided into fourths.

The layout is easiest when working from the crisp edges of the timber or from a drawing; hence the E scale is used. However if the edges are rough or non existant (bark or wane) then the center or middle is located and the M scale is used.The nominal width of the starting timber is found on either the E or M Scale and the distance from the edge to the line or the distance from the middle to the line is read from the inch scale.

For example: suppose you are working with a 10 inch x 10 inch timber and wish to make it a 8 sided timber. You know you must remove the corners, but how wide are each of the 8 faces?

You will see that the 10 on the E scale is opposite 2 7/8+ inches, resulting in 8 faces of 4 1/8 inch (10 - 2 x 2 7/8+ is about 4 1/8) when laid out on a 10 inch square. Note that 10 on the M scale is opposite 2 1/16 inches also giving faces of 4 1/8 inch.