Unknown numbers are usually represented by letters of the alphabet . ... However, different letters are also used, often hinting at the quantity they represent-- v for an unknown velocity, T for an unknown time, m or M for unknown masses and F for an unknown force. ... An example of case (#1) is the ratio between the length of a circle and its diameter, a universal constant appearing in many equations (for example, Kepler's equation in section (12a)), whose value to 11 decimals is 3.14159265359.. ...