The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926. It is often referred to as the Hubble tuning-fork diagram because of the shape in which it is traditionally represented. Hubble divided regular galaxies into 3 broad classes: ellipticals, lenticulars and spirals, based on their visual appearance. A fourth class contains galaxies with an irregular appearance. To this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy.