A comet that has been observed on more than one return to perihelion, allowing its orbital period to be established reliably. Originally the term referred to comets with periods up to 200 years, but this arbitrary cut-off is becoming obsolete as comets with ever-longer periods are recovered. Two sub-classes of periodic comet are now defined: short-period comets (also known as Jupiter-family comets), with periods less than 20 years, and intermediate-period comets (or Halley-family comets), with periods between 20 and 200 years; however, there is no real physical difference between the comets in each category. Most periodic comets have direct orbits with inclinations of less than 30°. The typical example has a period of seven years, perihelion 1.5 au, and inclination 13°. The names of periodic comets are prefixed by P/ (or D/ if they have disintegrated or disappeared), preceded by a number indicating the order in which their orbit was established (e.g. 1P/Halley, 2P/Encke, 3D/Biela, 109P/Swift–Tuttle). The periodic comet with the longest established period, 366 years, is Ikeya–Zhang. Most periodic comets are thought to come from the Kuiper Belt.