I just thought this was clever: In 1880 Chicago inventor Samuel Gross proposed printing scales on the edges of a map’s reverse side, so the user could measure the distance between two points by turning up a corner.

“In this figure I have placed the scales on the two edges forming the angle at the lower right-hand corner; but it is evident that they may be placed on any one or more edges or parts thereof of the reverse side of the map, as may be preferred.”