The most massive and most luminous Local Group galaxies are the two spirals Milky Way and M31 ( 1012 M, MV -21 mag). The third, less luminous and less massive Local Group spiral M33 does not have any known companions and belongs to the M31 subsystem. About two thirds of the Local Group galaxies are found within 300 kpc of the two spirals. The majority of these close companions are dwarf spheroidal and dwarf elliptical galaxies. The ensemble of dwarf irregular galaxies, on the other hand, shows little concentration toward the two large spirals (although the two most massive Local Group irregulars, the Large and the Small Magellanic Cloud (LMC and SMC), are close neighbors of the Milky Way and interact with it as well as with each other). This correlation between morphological type and distance from massive galaxies is also known as morphological segregation and may be to some extent a consequence of evolutionary effects.
Whether a galaxy should be considered a dwarf galaxy is somewhat arbitrary, and different authors use different criteria. For the purpose of this review all galaxies with MV > -18 mag will be considered dwarf galaxies, which results in 31 dwarfs, excluding only the three spirals and the LMC. We distinguish the following basic types of dwarf galaxies in the Local Group:
A few dwarf galaxies (Phe, LGS 3) are classified as ``transition-type'' objects and may be evolving from low-mass dIrrs to dSphs. These dIrr/dSph galaxies are found at distances of 250 kpc < DSpiral < 450 kpc. The Local Group does not contain blue compact dwarf galaxies, dwarf spirals, or massive ellipticals.