Use of 'an' or 'a' with acroynms

[stealthsquiggle]

OK, pedants, explain this one. The acronyms are geeky but the answer doesn't need to be.

SLA = Service Level Agreement

but it's a Service Level Agreement, but an SLA

TLA = Three Letter Acronym

but it's a Three Letter Acronym and a TLA

I think it just depends how the acronym is pronounced. An 'Ess-ell-ay' - starts with an E,
whereas TLA is A 'Tee-ell-Ay' starts with a T.

OK, interesting theory - let's test it:

an RDBMS
an MBA

OK those work

Come up with one that doesn't

Grima is right. It's to do with the pronunciation of the letter in the acronym rather than the pronunciation of the word that the letter stands for IYSWIM.