What is 19th letter greek alphabet?

The 19th letter of the Greek alphabet is called "Mu" (Μ, μ). Mu is the equivalent of the letter "M" in the English alphabet and has a numerical value of 40. Its uppercase form is written as Μ and its lowercase form is written as μ. Mu is derived from the Phoenician letter "Mem" and it is one of the most commonly used letters in the Greek language. In mathematics, mu is used to represent the mean or average, while in physics, it is used to represent the coefficient of friction.