It is immaterial which is so chosen, but usual practice is that it is one of the two longer sides.
In the parallelogram above, line $AB$ is considered to be the base.
The term is also used for the length of such a line.
- Rectangle is Parallelogram: a rectangle is a parallelogram all of whose angles are equal to a right angle.
- Altitude of Rectangle: the altitude of a rectangle is equal to one of its sides which is adjacent to its base.
- Results about parallelograms can be found here.
In the words of Euclid:
- Of quadrilateral figures, a square is that which is both equilateral and right-angled; an oblong that which is right-angled but not equilateral; a rhombus that which is equilateral but not right-angled; and a rhomboid that which has its opposite sides equal to one another but is neither equilateral nor right-angled. And let quadrilaterals other than these be called trapezia.