Same piece – different key

This topic has come up several times with our clients.  Sometimes our musicians are asked to play along with a singer, especially at wedding ceremonies:  it could be  a classical piece of music, or a popular song that has a special meaning for a couple.  The misunderstanding comes when we are asked to see if we have that piece in our repertoire.  Quite often we do, and in this case we did have the needed song.  However, speaking in musical terms, our version of the piece was in the key of D, and, as I found out by contacting the singer directly, her version was in the key of F, which means that these two versions could not be played together since they are in different keys.  
The concept of a piece of music being  in a specific “key” is not familiar to non-musicians.  Here is an attempt to present it in a different way. People’s voices are of different pitch, which is why we recognize the voices of our relatives and friends among strangers.  In music, we also have more than one pitch, more than one note. A piece of music or a song can start on ANY of the primary seven existing notes in music (there are more than seven, but I am keeping it simple). In order for us to play with a singer, we have to have the same version of a piece, one that starts and ends on the same note, otherwise our parts will “clash”, and not fit together in harmony.  Another way of saying it – the pattern of notes within the piece has to be the same, they have to match, and in terms of music theory – they have to be in the same “key”…