Error checking (kind of)

If you want to know the character (for error control purposes) of a Spanish identification document, just perform the modulo 23 operation over the DNI number. Then use the next table. That's it.

0	T	12	N
1	R	13	J
2	W	14	Z
3	A	15	S
4	G	16	Q
5	M	17	V
6	Y	18	H
7	F	19	L
8	P	20	C
9	D	21	K
10	X	22	E
11	B

And if you want to get a better idea about how CRC works you can visit this old post. There is a detailed Wikipedia article too.