Galaxies, the building blocks of the universe, range from simple to very complex in structure. In 1926, American astronomer Edwin Hubble developed a classification scheme of galaxies, more commonly known as the Hubble tuning fork. The basic ideas still hold today in helping astronomers understand the theory of galaxy evolution. The diagram pictured is split into two parts: elliptical galaxies, or ellipticals, and spiral galaxies, or spirals. Hubble assigned ellipticals numbers from zero to seven, while spirals were given letters. It is now believed that galaxy evolution is much more complicated than Hubble's classification scheme.