Deck-based random and the 7 pieces

Tetris features 7 pieces, the 7 tetrominos. They are:

T  

I  

L    J  

Z    S  

O  

Notice that L and J, and Z and S, are mirror images of each other. The other three, T, I, and O, are symmetric. Given the symmetry, any opening done with a given set of pieces can be done in mirror image, simply by substituting J for L and Z for S. That said, due to the starting positions of the pieces, not all openings are as good in mirror image--I will discuss specific examples in later posts.

Tetris uses deck-based random. The first 7 tetrominos one places are always the same, simply shuffled into a different order. Within the first 7 pieces, one will always get one T, one I, one L, one J, one Z, one S, and one O. Each subsequent deck of 7 pieces works the same way, shuffling the seven tetrominos and giving all seven in a random order.

The board is 20 squares high, and, more importantly, 10 squares wide. The first 7 pieces have 4*7 = 28 squares total,  meaning one can clear a bit more than two lines with them.

Some implications here:

• If you need a certain piece, you will always get it within the next deck. Builds which require a certain piece therefore come out okay. However, if you need three of the same piece, it'll be three long, lonely decks before you get them all. Try to create variety in the pieces you require.
• Every 7 pieces, one will get one T and one I. The highest damage single attacks are the T-spin and the Tetris, and the T and I respectively allow these attacks. Ideally, one would want to get a T-spin every T piece and a Tetris every I piece.
• The largest attack one can do with just the first 7 pieces is a T-spin double, clearing 2 rows (2*10 = 20 squares) with 8 squares left over. The soonest one can do it is on the 6th piece, since one needs an extra bit on top to register a T-spin. A (non-mini) T-spin single is possible on the 5th piece, though this opening is seldom used.
• Given that many openings can place the pieces in various orders, and one has the "hold piece" mechanic, there are certain setups one can "almost always" do.

Here's an example of a T-spin double opening. One can pretty much always do this, or a very similar opening, given I as the first piece.

I'll discuss more on the basic T-spin double openings in the next post.