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- Look at a ruler to see how small this is .
- Peter the great was a russian ruler .
- I need fifty sheets of plain paper and a ruler .
- They were welcomed at both places by the rulers .
- The country was bled white by her foreign rulers .
A ruler, sometimes called a rule or line gauge, is an instrument used in geometry, technical drawing, printing and engineering/building to measure distances and/or to rule straight lines. Strictly speaking, the ruler is essentially a straightedge used to rule lines, but typically the ruler also contains calibrated lines to measure distances.