I bought rules with both cause I'm not about to convert completely over. Just need to handle the times where going metric makes life easier. For instance, last year, in using "real" baltic birch ply for some cases, I thought it was easier to layout dados using mm scales as the thickness of the panels is manufactured in mm. The inch measurements that BB is sold by in the US are approximations.

Grabbed this (without permission) from the Allied Veneer Company website:
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Q: What thicknesses do you carry?
Thickness and unit quantities are defined as the following:
Thickness (mm) *Thickness (inches) Sheets per unit
3mm (1/8")- 130 sheets6mm(1/4")- 65 sheets9mm (3/8") - 44 sheets12mm (1/2") - 33 sheets15mm (5/8") - 26 sheets18mm (3/4") - 22 sheets
* Note - the Baltic Birch thicknesses in inches are approximate. For example, the most common thickness for drawer sides is 12mm or ½". However, a true half inch = 12.7mm. Baltic birch has been around long enough that the equivalency ratios stated above are considered the norm.
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