In everyday usage, a constellation is a traditional or recognizable group of stars in the night sky, or the region of the sky containing them. In technical usage these meanings have been separated, with groupings of stars being called asterisms, and the word constellation being reserved for a specific area of the celestial sphere with boundaries laid down by the International Astronomical Union (IAU). These areas mostly had their origins in Western-traditional asterisms from which the constellations take their names. When astronomers say something is in a given constellation they mean it is within those official boundaries. There are 88 officially recognized constellations, covering the entire sky which have grown from the 48 classical Greek constellations laid down by Ptolemy in the Almagest. Out of these 88 constellations, 12 compose the zodiac signs.