Well, any major keys relative minor is the minor key 3 half steps below that major key (If you have Bb major, then 3 half steps down is G, so Bb's relative minor is G minor). Anyway, this is more for reference than anything else. Say you're playing a song that goes, C - Am - F - G. This is in C Major, and the tonal center is C, but you can go ahead and solo with your A Minor pentatonic box without being out of key, because A Minor and C Major are relative (they both have the exact same notes). The only thing which you may need to watch out for is the notes you emphasize. Don't go emphasizing the A a lot in A Minor Pentatonic if the tonal center is C...
"You must stab him in the heart with the Bone Saber of Zumacalis... well, you could stab him in the head or the lungs, too... and the saber, it probably doesn't have to be bone, just anything sharp lying around the house... you could poke him with a pillow and kill him."
- Aqua Teen Hunger Force, The Universal Re-Monster
- Aqua Teen Hunger Force, The Universal Re-Monster