Paul - I'd just like to add that there is such a thing as "unequal bilateral" tolerancing, as well. Bilateral tolerancing is assumed to be equally distributed from the nominal feature size/location unless otherwise indicated either by text appearing below the feature control frame or dimension lines indicating both direction and amount of allowable deviation from nominal. An example would be a total allowable profile tolerance of 2.5mm with 1mm in one direction and 1.5mm in the other direction.

HTH,
Shane