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Comment by Someone | Hacker News Reader

I guess you are thinking of other orientations than straight up, as it is trivial to clear three lines with a T-piece:

   .....TTT..
   ......T...
   ......T...
   ..........

That's not a T piece. It has 5 blocks. The subtlety is about a board like

    .XXXXXXXXX
    ..XXXXXXXX
    .XXXXXXXXX

and if you can put a T piece inside of the gap on the left.

Oops. My mind is more pentomino-oriented, I guess.

All seven pieces ("tetrads", from "tetromino") in standard tetris have 4 blocks, not 5.