The thick disk is one of the structural components of several galaxies, including the Milky Way. It was first proposed as a unique galactic structure, different from the thin disk and the halo in an 1983 article by Gilmore & Reid. It is supposed to dominate the stellar number density between 1 to 5 kiloparsecs (3.3 to 16.3 kly) above the Galactic plane and is composed almost exclusively of older stars. Its chemical composition and kinematics (those of the stars comprising it) are also said to set it apart from the thin disk. Stars within the thick disk may be called old-disk stars. Compared to stars in the thin disk, these typically have a significantly lower levels of metals—that is, the abundance of elements other than hydrogen and helium.