In the French astronomer Gérard de Vaucouleurs' revised Hubble Sandage (VRHS) system of galaxy morphological classification, the Triangulum Galaxy is classified as type SA(s)cd. The "S" prefix indicates that it is a disk-shaped galaxy with prominent arms of gas and dust that spiral out from the nucleus—what is commonly known as a spiral galaxy. The "A" is assigned when the galactic nucleus lacks a bar-shaped structure, in contrast to "SB" class barred spiral galaxies. American astronomer Allan Sandage's "(s)" notation is used when the spiral arms emerge directly from the nucleus or central bar, rather than from an inner ring as with an (r)-type galaxy. Finally, the "cd" suffix represents a stage along the spiral sequence that describes the openness of the arms. A rating of "cd" indicates relatively loosely wound arms.