Thieves' do extra damage on a backstab based on their level as listed in the table. This multiplier is only applied to the first attack on a humanoid creature after coming out of hiding. In AD&D, the multiplier is applied after adding strength bonuses but before adding magical weapon bonuses. In Baldur's Gate, the multiplication will be done before adding the strength bonus due to the easy availablity of Potions of Giant Strength which were making thieves' backstabbing too powerful.

Thief Level     Damage Multiplier
   1-4                x2
   5-8                x3
   9-12               x4
   13+                x5