The base 16 notational system for representing real numbers. The digits used to represent numbers using hexadecimal notation are 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, and F. The following table gives the hexadecimal equivalents for decimal numbers from 1 to 30.
| 1 | 1 | 11 | B | 21 | 15 |
| 2 | 2 | 12 | C | 22 | 16 |
| 3 | 3 | 13 | D | 23 | 17 |
| 4 | 4 | 14 | E | 24 | 18 |
| 5 | 5 | 15 | F | 25 | 19 |
| 6 | 6 | 16 | 10 | 26 | 1A |
| 7 | 7 | 17 | 11 | 27 | 1B |
| 8 | 8 | 18 | 12 | 28 | 1C |
| 9 | 9 | 19 | 13 | 29 | 1D |
| 10 | A | 20 | 14 | 30 | 1E |
The hexadecimal system is particularly important in computer programming, since four bits (each consisting of a one or zero) can be succinctly expressed using a single
hexadecimal digit. Two hexadecimal digits represent numbers from 0 to 255, a common
range used, for example, to specify colors. Thus, in the HTML language of
the web, colors are specified using three pairs of hexadecimal digits RRGGBB,
where 
is the amount of red, 
the amount of green, and 
the amount of blue.
In hexadecimal, numbers with increasing digits are called metadromes, those with nondecreasing digits are called plaindrones, those with nonincreasing digits are called nialpdromes, and those with decreasing digits are called katadromes.
