Usually on one side they are centimetres (numbered) with millimeters (marked but not numbered). On the other side there are inches (numbered) with eighths or tenths or sixteenths or even thirty-secondths (marked but not numbered).
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With a "ruler" or "tape measure" calibrated in imperial units
22 - Apex !
You measure length in many ways: using a ruler or yardstick, using a string or tape measure, using a machine to drive or fly over a distance, or using a satellite. Length is measured in units such as centimeters, meters, kilometers, or inches, yards and miles.
You can convert units of length to units of length, and units of area to units of area. But it doesn't make sense to convert between units of length and units of area.
None. Area cannot be measured in length units - only in squares of such units.