The hexadecimal number system is a base-16 numbering system, meaning it uses sixteen distinct symbols to represent values. Since the ordinary decimal digits 0 to 9 only provide ten symbols, hexadecimal borrows six letters of the alphabet to represent the remaining values.
Counting upward from 9, the next value (equivalent to decimal 10) is represented by the letter A, decimal 11 by B, decimal 12 by C, decimal 13 by D, decimal 14 by E, and decimal 15 by F. This gives the complete set of sixteen hexadecimal digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F. The sequence therefore runs from 0-9 and A-F, which completes the sixteen required symbols.
A letter such as D or E marks only the fourth or fifth letter in the sequence, not the last one, so stopping there would leave the system with fewer than sixteen symbols. Only F closes the full run from A through F.
Exam tip: remember that hexadecimal always has exactly sixteen symbols, so if you are ever unsure where the letter sequence ends, count six letters starting from A; the sixth letter, F, is always the final hexadecimal digit.
