Two related quantities *x* and *y* are called **proportional** (or **directly proportional**) if there exists a constant non-zero number *k* such that

*y*=*kx*

*y*~*x*.

To test whether *x* and *y* are proportional, one performs several measurements and plots the resulting points in a Cartesian coordinate system. If the points lie on (or close to) a straight line passing through the origin (0,0), then the two variables are proportional, with the proportionality constant given by the line's slope.

The two quantities *x* and *y* are **inversely proportional** if there exists a non-zero constant *k* such that

*y*=*k*/*x*

**See also:** proportional font, proportional representation