Base-20 (also known as vigesimal) number system uses 20 symbols to code information. Most commonly, these symbols are, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 'A to J' (or alternatively 'a to j').
Base-7 (aka septenary) number system uses seven symbols to code information. Most commonly, these symbols are, 0, 1, 2, 3, 4, 5 and 6.