Possible Clues:

• Ruler divs
• Metric ruler units, for short
• 1 in. = 2.54 ___
• Ruler meas
• Mil. trials
• Divs. on some rulers
• Ruler amts.
• 30.48 of them fit in a ft
• There are 91.44 in a yd.
• The 2.54 in an in.