how exactly does jet determine which key to use?

Hi,

I'm frequently confused by the key choice of Jet. When for instance, I do a row selection with filters on 3 specific fields, sometimes Jet will choose to use a key in which only field3 is included while there is a key that covers both field1 and field2 which would be the most logical key to use performance wise. Only after switching the order of the keys, trial and error wise, I seem to be able to eventually get it to pick the correct key. I've tried the '+' sign and tried to change the order of the filters, but in some cases Jet will insist on using a slow and illogical key.

What are the standard rules Jet uses to determine which key to use?

P.s. I've read through the other thread covering this question, but couldn't find the standard rules Jet uses. I'm using the lastest version of Jet.