 If several pairs of variables share the same direct proportionality constant, the equation expressing the equality of these ratios is called a proportion, e.g., (for details see Ratio).Given two variables "x" and "y", "y" is directly proportional to "x" if there is a non-zero constant "k" such that

The relation is often denoted using the symbols "∝" (not to be confused with the Greek letter alpha) or "~"