A square is a quadrilateral having all its sides equal, and all its angles right angles. The diagonal of a square is equal to the square root of twice the square of its sides: and the side of the square is equal to the square root of half the square of its diagonal.
The diagonal is a right line drawn across a quadrilateral figure, from one angle to another. The sum of the squares of the two diagonals of every parallelogram is equal to the sum of the squares of the four sides.
A parallelogram is a quadrilateral, whose opposite sides are parallel.
A rectangle is a parallelogram having four right angles.
A rhomboid is an oblique-angled parallelogram.
A rhombus, or lozenge, is a quadrilateral, whose sides are all equal but its angles oblique.
A trapezium is a quadrilateral, which has none of its sides parallel to each other.
A trapezoid is a quadrilateral, which has only two of its sides parallel.
Polygons are plane figures bounded by more than four sides.
A regular polygon has all its sides, and angles equal.