In the days of slide rules, before electronic computers, many engineering problems were solved using nomograms (also known as nomographs). In its simplest form, a nomogram is a kind of graph that shows the relationship between three variables in a very special way: by drawing a line between values for any two variables, you find the value of the third.

In these days of digital computing, the use of nomograms have faded, and they now are mostly of historical interest, but like many things of historical interest, they are of personal interest to me. Hence, I was delighted to find this reference which provides a reasonable introduction, and also a good list of reference books.

Dead Reckonings » The Art of Nomography I: Geometric Design.