The thickness of the material is called gauge.  With the constant change of the Industrial Revolution it became necessary to standardize.  J. R Brown started making and selling tools in 1833 and invented the first micrometer.   In 1853 the firm of J. R. Brown and Sharpe of Providence R.I. helped to set standards with their American Standard Wire Gauge (there is also an English Standard Gauge).  It serves to measure thickness of sheet metal as well as wire.  Here is one from that period.

The numbers start at 0 and go to 36

Modern Sheet metal workers still use them and refer to them as go no go gauges.  If the tool goes onto the material it is the right thickness.  If it does not go onto the next it is the larger size.