The term "double star" is often used synonymously with "binary star"; however, double star can also mean "optical double star". Optical doubles are so called because the two stars appear close together in the sky as seen from the Earth; they are almost on the same . Nevertheless, their "doubleness" depends only on this optical effect; the stars themselves are distant from one another and share no physical connection. A double star can be revealed as optical by means of differences in their parallax measurements, proper motions, or radial velocities. Most known double stars have not been studied adequately to determine whether they are optical doubles or doubles physically bound through gravitation into a multiple star system.