What is the difference between a plain scale and a diagonal scale?

Answer 1

The plain scale is used for simple scales, scales that do not have many subdivisions.

while Diagonal scale

There is a limit to the number of divisions that can be constructed on a plain scale.Try to divide 10 mm into 50 parts; you will find that it is almost impossible. The architect, cartographer and surveyor all have the problem of having to subdivide into smaller units than a plain scale allows. A diagonal scale allows you to divide into smaller units.