Triangulum is a small northern constellation, one of the forty-eight listed by Ptolemy in the second century and retained among the modern eighty-eight. Its handful of stars form a narrow elongated triangle south of Andromeda, and the Greeks knew it as Deltoton for its resemblance to the letter delta; some ancient writers linked it to the Nile Delta or to the island of Sicily. Faint though it is, it contains the Triangulum Galaxy, a member of the Local Group visible in dark skies. In astrology the constellations of the sky are distinct from the zodiac signs that share some of their names, and a small figure such as Triangulum carries little of the symbolic weight the zodiacal constellations acquired. These are the features of the constellation as the classical and modern astronomical traditions describe it, separate from any astrological reading of the region.
Ptolemy's forty-eight is not the whole story of how the modern eighty-eight was assembled: the gaps between his figures, the faint, awkward spaces his hunters, animals and heroes left uncovered, were filled only later, by Johann Bayer in 1603, by Petrus Plancius and Jakob Bartsch in the early seventeenth century mapping the newly explored southern sky, by Johannes Hevelius in 1687, and by Nicolas-Louis de Lacaille in the 1750s, whose eighteenth-century southern constellations were named for instruments of the new science, telescope, microscope, furnace, rather than for any mythological figure. Triangulum's small neighbour, Triangulum Minor, was one of Hevelius's additions and did not survive into the standardised modern list, one of several constellations proposed after Ptolemy that later fell out of use entirely.