Attack Rating and Armor Rating

To Hit

D20 + Attack Rating >= Armor Rating 

Attack Rating

Fighter +1 and +1 per level (max 16)

Monk +1 per level (max 12 weapon/ max 21 hands)

Cleric +2 per 3 levels (max 12)

Thief +1 per 2 levels (max 10)

Magic User +2 per 5 levels (max 8)

Monsters +2 and +1 per hit die (max 20)

Armor Rating

Armor Class	Armor Rating
10	10
9	11
8	12
7	13
6	14
5	15
4	16
3	17
2	18
1	19
0	20
-1	21
-2	22
-3	23
-4	24
-5	25
-6	26
-7	27
-8	28
-9	29
-10	30