The metal strip on a wooden ruler is there so that you can draw a crisp, straight line. In addition, the metal strip will stop a felt-tip marker from smearing on paper. Each type of ruler actually has different features to mimic this same result:
- A stainless steel ruler is a piece of very thin metal raised up by the cork backing underneath (liftoff.) When you draw a line with a felt-tip marker, ink will not spear because of liftoff.
- Aluminum rulers have an embedded metal strip, or drawing edge on one side to prevent smearing.
- Most plastic rulers are not recommended for use with felt-tip markers and should only be used with pencil or pen.