OK, I think I have confused myself and this is probably stupid. A Lvl1 Priest has a Thac0 of 20 and a progression of 2/3. I can figure out Thac0 down to 20th Lvl, but I am having problems with Thac-1 thru Thac-10. Would it be something like:
THAC0 is a measurement of your accuracy in combat, to get your required roll, you simply subtract your target's AC:
My THAC0 is 16
Vs AC 5 I need to roll a 16 - 5 = 11 or below.
Vs AC 0 I need to roll a 16 - 0 = 16 or below.
Vs AC -5 I need to roll a 16 - -5 = 21 or below. (a natural 20 always hits).
Strange how I find it easier to think about it this way, rather than aliasmask's. I want to know what I need to roll before I hit, so I can punch the air jubilantly or sulkingly pass my d20 over the table.
We had a very handy chart back in the day (first appeared on the custom Barbarian sheet, go figure).
Its a chart that lists all the AC values (10 through -10) and had a slot for your thac0, and your total ToHit modifier. Every time you gained a modifier or a level, you would just do the math ahead of time and fill in each chart.
Then you'd know the exact number you needed to roll on your dice to hit any AC from the chart.
In example, Strength 18/00, if I remember correctly is a +3 To Hit modifier.
So a level 1 'Warrior' with thac0 20, and 18/00 strength would have a similar chart:
To Hit AC 10 = 7 (that's 20 - AC 10 - 3 from strength)
To Hit AC 9 = 8
To Hit AC 8 = 9
To Hit AC 7 = 10
And so forth. Speaking Maptools, I made a frame to display this chart, filling in all relevant details, to my players when they opened their character sheet. Just thought I'd chime in.
"An arrogant person considers himself perfect. This is the chief harm of arrogance. It interferes with a person's main task in life - becoming a better person." - Leo Tolstoy