A Bayer designation is a way of naming stars in a constellation. In was introduced in 1603 by Johann Bayer in his star atlas, Uranometria and categorizes stars by their relative brightness to other stars in the constellation. The designation of a star consists of two components, the first a Greek letter and the second, the genitive form of the constellation name. The brightest star (as seen from Earth is designated the alpha star in the constellation, the second brightest the beta star and so on up the Greek alphabet. For example, Gamma Orionis is the third brightest star in the constellation of Orion (gamma is the third letter in the Greek alphabet). The system was later extended to include Latin letters in addition to Greek ones. Together with the extension, it has been used to name roughly 1,300 stars.
Though most stars with Bayer designations follow the rule above there are some exceptions. This is usually due to two star having similar magnitudes and the dimmer star being measured as the brighter one, but the original designation stuck. Some stars have two designations (known as double designations) such as Beta Tauri (Gamma Aurigae) and Alpha Andromedae (Delta Pegasi).