ABSTRACT
The definition of length appears simple, but there are several problems in practice. The length of an object is the number of standard units which can be laid in a straight line along or beside the object. The number need not be a whole number, and can be given to any degree of accuracy. In school, children frequently use informal units such as matches or straws before being introduced to the SI units. The metre is the standard unit for length. The perimeter is the distance around the edge of a shape. So the perimeter of a square is four times the length of one of its sides. Although Pythagoras' theorem is, in origin, one concerning the area of squares, it is frequently used to calculate lengths. The theorem states that in a right-angled triangle the square on the hypotenuse is equal to the sum of the squares on the other two sides.
