Hi everyone ! I was interested in starting a new game of BG1, since I never finished the one I started long ago, but I remember you get to choose weapon proficiencies during character creation. Problem is, I never really knew what was the difference between the proficiencies. I looked it up on the internet, but I mostly stumbled upon vague, specific answers. Could someone shed some light on this ?

Level of proficiency Slots spent Bonus THAC0 Bonus damage Melee Attacks per round
Unskilled 0 -1 -1 1
Proficient 1 0 0 1
Specialized 2 +1 +2 3/2
Master 3 +3 +3 3/2
High Master 4 +3 +4 3/2
Grand Master 5 +3 +5 5/2

For BGEE, the last number is 2 (grand master only gives another half attack instead of 1)

So the difference is only cosmetic ? As well as a difference of availability ? That's... weird. And disappointing. Thanks for the answer.

How is that cosmetic? It's a huge difference in a warrior's battle prowess. And being completely unskilled actually penalizes most classes much worse than -1.

You are talking about different weapons?

Besides availability, there is a major difference: damage type
there are 4 types of physical damage, blunt (e.g. morning star, mace), slashing (e.g. sword, axe), piercing (e.g. spear) and missile (bows and crossbows)
different armor have different value for each type of damage. Some creatures are more resistant to some types of damage.
Usually blunt is best

There is another difference: weapon range
quarterstaff and some other weapon has a longer reach. But I am afraid it matters little.

Some corrections:

If you are non-proficient, fighter-type get a -2 penalty, single-class mages (and I believe sorcerers) get -5, and other characters get -3. Furthermore, fighter-types don't get the extra (half-)attack for being a high enough level without proficiency.

In Baldur's Gate 2 (non-EE) (or if you are playing 1 with TuTu or Trilogy), the table looks like:

Level of proficiency Slots spent Bonus THAC0 Bonus damage Melee Attacks per round
Unskilled 0 -3* 0 1
Proficient 1 0 0 1
Specialized 2 +1 +2 3/2
Master 3 +2 +2 3/2
High Master 4 +2 +3 3/2
Grand Master 5 +2 +4 3/2

* Depends on class

Also, I am pretty sure that the last number is 2 in AD&D 2e, not 5/2. (Maybe some people misinterpret the table in the book that introduced Grand Mastery?)

Yeah, that's what I wanted to know, thanks. Doesn't work exactly as I would have wanted it to, but I guess it'll have to do. ^^

It is 5/2 in PNP, and is implemented as so in BG, IWD
It is never 2 except in BGEE and BGEE2