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When you use your body to measure something,be it your entire body or just parts of it.
Men tend to use their penises as human rulers.

Chris knew each of his feet was exactly twelve inches,so instead of using a tape-measure to measure the wall,he walked next to the wall putting one foot next to the other.

Jack used his 12-incher to measure the ruler and make sure it was exactly 12 inches.

Arwin knew most markers are 6 inches because the were the same size as his dick.

I had trouble writing examples for this definition,so I used my three-inch long middle fingers and found out the average business card is about 3.25 by 2 inches.
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