E. Hexadecimals

"Regular" numbers are based on the base 10 system, that is, there are ten symbols (what we call "numbers"): 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. To represent numbers greater than 9, we use a combination of these symbols where the first digit specifies how many ones, the second digit (to the left) specifies how many tens, and so on.

In the hexadecimal system, which is base 16, there are sixteen symbols: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, a, b, c, d, e, and f. To represent numbers greater than f (which in base 10 we understand as 15), we again use a combination of symbols. This time the first digit specifies how many ones, but the second digit (again, to the left) specifies how many sixteens. Thus, 10 in the hexadecimal system means one sixteen and no ones. In the base 10 system, it'd be 16.

In (X)HTML and CSS, hexadecimal numbers are used to define colors (see page 126) and to insert symbols (see page 336). While you can convert hexadecimal numbers by hand, I've also included a table to help you quickly look up a number's hexadecimal equivalent.

Hexadecimal Equivalents

#	Hex.	#	Hex.	#	Hex.	#	Hex.	#	Hex.	#	Hex.	#	Hex.	#	Hex.
0	00	32	20	64	40	96	60	128	80	160	A0	192	C0	224	E0
1	01	33	21	65	41	97	61	129	81	161	A1	193	C1	225	E1
2	02	34	22	66	42	98	62	130	82	162	A2	194	C2	226	E2
3	03	35	23	67	43	99	63	131	83	163	A3	195	C3	227	E3
4	04	36	24	68	44	100	64	132	84	164	A4	196	C4	228	E4
5	05	37	25	69	45	101	65	133	85	165	A5	197	C5	229	E5
6	06	38	26	70	46	102	66	134	86	166	A6	198	C6	230	E6
7	07	39	27	71	47	103	67	135	87	167	A7	199	C7	231	E7
8	08	40	28	72	48	104	68	136	88	168	A8	200	C8	232	E8
9	09	41	29	73	49	105	69	137	89	169	A9	201	C9	233	E9
10	0A	42	2A	74	4A	106	6A	138	8A	170	AA	202	CA	234	EA
11	0B	43	2B	75	4B	107	6B	139	8B	171	AB	203	CB	235	EB
12	0C	44	2C	76	4C	108	6C	140	8C	172	AC	204	CC	236	EC
13	0D	45	2D	77	4D	109	6D	141	8D	173	AD	205	CD	237	ED
14	0E	46	2E	78	4E	110	6E	142	8E	174	AE	206	CE	238	EE
15	0F	47	2F	79	4F	111	6F	143	8F	175	AF	207	CF	239	EF
16	10	48	30	80	50	112	70	144	90	176	B0	208	D0	240	F0
17	11	49	31	81	51	113	71	145	91	177	B1	209	D1	241	F1
18	12	50	32	82	52	114	72	146	92	178	B2	210	D2	242	F2
19	13	51	33	83	53	115	73	147	93	179	B3	211	D3	243	F3
20	14	52	34	84	54	116	74	148	94	180	B4	212	D4	244	F4
21	15	53	35	85	55	117	75	149	95	181	B5	213	D5	245	F5
22	16	54	36	86	56	118	76	150	96	182	B6	214	D6	246	F6
23	17	55	37	87	57	119	77	151	97	183	B7	215	D7	247	F7
24	18	56	38	88	58	120	78	152	98	184	B8	216	D8	248	F8
25	19	57	39	89	59	121	79	153	99	185	B9	217	D9	249	F9
26	1A	58	3A	90	5A	122	7A	154	9A	186	BA	218	DA	250	FA
27	1B	59	3B	91	5B	123	7B	155	9B	187	BB	219	DB	251	FB
28	1C	60	3C	92	5C	124	7C	156	9C	188	BC	220	DC	252	FC
29	1D	61	3D	93	5D	125	7D	157	9D	189	BD	221	DD	253	FD
30	1E	62	3E	94	5E	126	7E	158	9E	190	BE	222	DE	254	FE
31	1F	63	3F	95	5F	127	7F	159	9F	191	BF	223	DF	255	FF

So, to use this chart, imagine you want to find the hex value for a color with 35% red, 0% green, and 50% blue. The percentages are relative to 255, so 35% x 255 = 89. Now, find the hexadecimal equivalent of 89, near the bottom of the third column above. So for red, we have 59. Green is easy; 0 = 00. For blue, we again have to multiply the percentage by 255 to get the numerical value. 50% x 255 is 127 (more or less). Then find the hex value for 127, at the very bottom of the fourth column. So, the blue is 7F. The final step is to write it all together: #59007F, which will get us a fine dark purple, precisely 35% red, 0% green, and 50% blue.