Ruler.

A ruler is a measuring tool used to accurately measure the length, width or height of an object. It typically consists of a straight and flat piece of material, such as plastic or metal, with markings along its length that indicate units of measurement such as inches or centimeters. The ruler is placed against the object being measured and the correct measurement is read from the markings on the ruler. Rulers are commonly used in classrooms, offices, and workshops and come in various sizes and shapes to suit different purposes.