A ruler is a flat tool used for drawing straight lines and measuring length. Most rulers are made of wood, plastic, or metal, and they're marked with inches on one edge and centimeters on the other. When you need to draw a perfectly straight line in your math notebook or measure whether your poster will fit on the wall, you reach for a ruler.

The word also means a person who governs a country or territory. A ruler might be a king, queen, emperor, or dictator: someone with the power to make laws and major decisions that affect everyone in their domain. Queen Elizabeth I was a ruler of England. Julius Caesar was a ruler of Rome. Some rulers govern wisely and fairly, while others rule through force and fear.

When reading, context makes the meaning obvious. If someone says, “I need a ruler for this geometry problem,” they want the measuring tool. If your history book mentions “the ruler of ancient Egypt,” it's talking about the pharaoh.