Thomas Tallis, you magnificent bastard.  This piece is actually a canon, called a “six in two.”  This means it uses six voices to produce a double canon.  This means that the top two voices are playing off each other while the bottom voices are doing their own thing.  You can track in the score how the second soprano follows the first soprano’s lead.  It’s one of my favorite Tallis pieces – which is saying something, since I carry a serious torch for the guy.  I love how it slowly builds to the two-minute mark, plus the interplay of the soprano lines between 2:46 and 2:56.  But the most ingenious part of the piece is how it ends on a question, by which I mean the chord doesn’t resolve back to the tonic (or starting chord of the piece); it ends on the fifth.  Miserere nostri – Have mercy on us, Lord.  It’s a request.  That the piece ends without resolution leaves space for that request to be answered.  It just brings tears to my eyes every time.