A parallelogram is a four-sided shape. The opposite sides of a parallelogram are always parallel, which is an important concept in geometry. Opposite sides are also the same length. When two lines, or sides of a shape, are parallel, they are equal distances apart from each other. This concept is useful when determining the angles within a shape. Several shapes are parallelograms: squares, rectangles and rhombuses.
A parallelogram is also used in geometry to illustrate how to calculate the angles of each corner. A parallelogram has four corners, each with an angle measurement. In a parallelogram, the opposite angles are always equal to each other. When determining if a shape is a parallelogram, the angles are measured. If one pair of opposite angles is equal, the shape must be a parallelogram.
When the measurements of two angles add up to 180 degrees, the pair of angles is considered supplementary. In a parallelogram, a set of consecutive angles that is supplementary. This means that two angles, next to each other, have angle measurements that add up to 180 degrees exactly, when combined. A square fits into the category of a parallelogram, because it consists of four congruent angles. Each angle in a square is exactly 90 degrees, and two consecutive angles in a square are 180 degrees exactly.
Another geometric concept learned through parallelograms is diagonals. Parallelograms illustrate this concept and the concept of congruent triangles. To illustrate this, divide a parallelogram from one corner to another. Next, divide it from the opposite corners. In a parallelogram, these diagonal lines will intersect directly in the middle of the shape. A single diagonal divider of the parallelogram will create two congruent triangles, meaning they are identical in shape and size.