Channel letters are made of plastic or metal and used as exterior signage on commercial or public buildings. They are often lit internally. Unlit 3D letters are also applied to panels and called dimensional letters. A channel letter is any number, letter or character used in the sign. The major parts of channel letters are the face, return, back, and lighting element. The returns, or letter sides, are made of aluminum sheeting. The face and back are usually acrylic but can also be aluminum, or nonexistent. Because they are highly flexible, channel letters signs are available in an array of colors, fonts, and sizes.