Periodic comets sometimes have the same name, for example there are nine Shoemaker-Levy comets and twenty-four NEAT comets. The Minor Planet Center, a part of the International Astronomical Union gives comets a different number prefix or uses the full title such as 181P/Shoemaker-Levy and 192/Shoemaker-Levy which are both "Comet Shoemaker-Levy". Sometimes an informal numbering system is used for periodic comets so 181P is Comet Shoemaker-Levy 6 and 192P is Comet Shoemaker-Levy 1. Because non-periodic Shoemaker-Levy comets are included in the number sequences there appear to be gaps in the numbers, for example C/1991 B1 is between Shoemaker-Levy 2 and 3, C/1991 T2 is between 5 and 6, C/1993 K1 and D/1994 E2 after Shoemaker-Levy 9.