The term "Gregorian" primarily refers to the Gregorian calendar, which is the calendar system most widely used today. Introduced by Pope Gregory XIII in 1582, it was designed to correct inaccuracies in the previous Julian calendar. The Gregorian calendar consists of 12 months and a total of 365 days, with a leap year every four years to account for the extra quarter of a day in the Earth's orbit around the sun. In addition to the calendar, "Gregorian" can also refer to Gregorian chant, a form of plainchant used in the Roman Catholic Church. This style of music is characterized by its monophonic, unaccompanied vocal lines and is named after Pope Gregory I, who is traditionally credited with organizing the chant repertoire. Both the calendar and the chant have had significant cultural and historical impacts.