The thick disk is one of the structural components of about 2/3 of all disk galaxies, including the Milky Way. It was discovered first in external edge-on galaxies. Soon after, it was proposed as a distinct galactic structure in the Milky Way, different from the thin disk and the halo. It is thought to dominate the stellar number density between 1 and 5 kiloparsecs above the galactic plane and, in the solar neighborhood, is composed almost exclusively of older stars. For the Milky Way, the thick disk has a scale height of around 0.6–1.1 kiloparsecs (2.0–3.6 kly) in the axis perpendicular to the disk, which is 3-4 times larger than the thin disk, and a scale length of around 1.9–2.3 kiloparsecs (6.2–7.5 kly) in the horizontal axis, in the direction of the radius. Its stellar chemistry and stellar kinematics also set it apart from the thin disk. Compared to the thin disk, thick disk stars typically have significantly lower levels of metals—that is, the abundance of elements other than hydrogen and helium.