Base Attack Bonus, Multiclassing, And Iterative Attacks

[Ninjaxenomorph]

I have several characters who are multiclassing or taking levels in a PrC in Pathfinder Society. It has never been made clear to me how it stacks with other classes, exactly.

Case 1: I have a samurai who will eventually take levels in Mammoth Rider; both classes have straight BAB. As per the description of BAB, he still gets bonus attacks, just as if he had never multiclassed.

Case 2: I have a magus who recently started multiclassing into cleric. Both have 3/4 BAB progression. The character is a Magus 7/Cleric 1, getting a +5 from the magus and +0 from the cleric currently, I believe. Does it actually mean that I have the normal BAB of an 8th level 3/4 BAB class, or does it stack normally, with the character not getting another attack until the cleric's BAB reaches +1, giving him a +6,+1 total?

Case 2.5: ... Then what happens when he multiclasses to Mystic Theurge, a 1/2 BAB PrC?

[AerynTahlro]

You simply add the columns from the current position.

A Magus 7 has a BAB of +5, Fort +5, Ref +2, Will +5.
Add one level of Cleric and these numbers increase by +0, +2, +0, +2, respectively.

[Shar Tahl]

You add the BABs together from the class tables of each class to get a total BAB. After every 5, you gain an iterative attack as normal. This is the same process that happens for saving throws as well. adding a 3/4 class always slows down the BAB progression. Only two full BAB progress seamlessly

[Lord Pendragon]

What they said. As you seem to have already realized, BAB suffers greatly from excessive multiclassing, unless all your classes are full BAB classes. Anything less and that first level is going to drop your BAB by 1 each time, with the loss getting worse and worse at higher levels.

Hopefully you have the spells/abilities to compensate if you plan to go into melee with that magus/cleric/theurge, because his BAB will be terrible. Your Samurai/Mammoth Rider, however, will be just fine.

[Ninjaxenomorph]

Well, unless he starts playing modules or something, he's not gonna get to the urge, that's just theoretical. And yes, he does have spells to compensate, thankfully. By next level, just the low-level buffs will equal out the loss.

[Kazaan]

If you're going to dip into a 3/4 BAB class, it may be a boon to devote a full 4 levels to it to avoid "missing out" on potential BAB, unless your other class is full-BAB. For your Magus/Cleric example, both are 3/4 BAB classes. If you are a Magus-8, you have +6 BAB. However, if you are a Magus-7/Cleric-1, you only have +5 BAB. If you were a Magus-7/Fighter-1, you'd still have +6 BAB. Honestly, I think it'd be better, though, if a mid-BAB and low-BAB classes actually do get fractional BAB and carry the fractions so that even a Magus7/Cleric1 would still get +6 BAB (7*3/4 + 3/4) = 6