WHen I worked as a die-maker in the middle of the continent between the oceans ... north of 'ole' and and south of 'eh' we called them 'scales' because they were used to scale drawings / get rough measurements of steel before machining.
In our business, the drawings were often 1:1 so you could get a quick reality check simply my holding the scale against it on the chance that the indicated dimension was, in fact, a brain fart.
I still have & use my pocket scales (6") with a wide variety of graduations (fractional inch, decimal inch and metric).
Even if you don't need the little lines & number, their edges are usually pretty darned straight, making them a relatively inexpensive / moderately accurate straight edge for gaging flatness.
Bill