Alfredo,
This is a praise & worship scenario, with (possibly) a new worship leader playing a song you recognize and know, but in a new key. You never played with this person before and they didn't dicuss which songs they would sing and in what key.
Think of this as an impromptu jam session, where you have heard the song before, know the lyrics, etc., but don't know what key the singer will sing in. Now, how do you jump in and start playing without hitting keys on the keyboard?
Lukitoh, jump in if I am off base here.
Starkeeper
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I play Roland EM20 and Yamaha PSR550