Triazalo benzodiazepine numbering scheme?

[NJC]

Hello. New to the boards.

I've just recently been researching various chemical compounds out of curiosity and have come across a naming scheme that I just can't make sense out of. The 1,4-triazalo benzodiazepines are confusing me a little bit.

Eg.) 8-chloro-1-methyl-6-phenyl-4H-[1,2,4]triazolo[4,3-a][1,4]benzodiazepine
eMolecules Source

I don't understand why the numbering scheme is just a little bit off compared to regular 1,4-benzodiazepines, such as Clonazepam or Oxazepam. Their chloride group is at the '7' position on the benzene ring, but on the triazalos it's switched to the '8' position. Same thing for the attached phenyl functional group. And what's with the methyl group at the '1' Nitrogen position as opposed to ascribing it to the triazalo structure? I know it has something to do with the actual triazalo structure and the IUPAC naming conventions, but I'm clueless as to what (I'm probably confusing myself more than I need to).

Basically, if I would come across a numbering diagram for the triazalo benzodiazepines, sort of like this diagram (first one on the left), what would it look like? I know it's probably something basic, but I guess my ego can rest for a lil' bit and resort to asking for help.

 

[Acyl]

You count all around that three ringed fused system, labeling each carbon that isnt at a fused center with a number. The phenyl group is a substituent and the carbons that are common to two rings are irrelevant.

 

[NJC]

Ah, makes sense. Thanks a lot, acyl.

 

[NJC]

I have another quick question about the alprazolam IUPAC name. What exactly does the [4,3-a] necessarily refer to? I'm assuming the 'a' stands for alpha, but I'm not sure if it's referring to the triazolo group or the diazepine ring structure.

Any help would be much appreciated.