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.