Tetris/Advanced strategies

Stack building
Throughout play, you must wait for the opportunity to clear lines to present themselves. In the mean time, you must place pieces in such a way that they: The first objective is a bit easier than the second, since the second requires more planning to achieve. In general, you should always have one portion of your stack available to accommodate an O-piece by having a two-square flat section on top of your stack. Likewise, there should be places where an S or Z piece and be placed. T, J and L pieces are usually a little easier to accommodate, and I pieces typically fit anywhere. However, it's not a good idea to place I pieces in the second column for the left or right side of the matrix if it's going to create a narrow gap that only another I piece can fit into.
 * fit uniformly into your stack so that they leave no holes or gaps (if at all possible) and
 * present the maximum number of opportunities for any other piece to be placed on the stack without leaving holes or gaps.

Garbage and line clearing
Throughout play, either as a result of making mistakes, presetting the height in a "B-Type" game, or when you are "attacked" in a multi-player game, you will find yourself in situations where you must clean up garbage; rows that have one or more holes in them that are buried beneath other rows. In order to deal with this, you must: For the second point, I pieces are useful, as well as T pieces which can be rotated to either side to prevent compounding the problem that you are trying to fix.
 * clear the rows above the garbage rows in order to gain access to them and clean them up but
 * try not to clear the rows with pieces that leave blocks remaining above the holes and gaps that you are trying to clear.

Realize that when clearing lines, you don't necessarily have to clear contiguous lines with one piece. It's possible to clear two separated lines with L or J pieces, and only two or three lines with I pieces. Even O, S and Z pieces can be used to clear lines that aren't necessarily at the top of the stack. Look for these opportunities and exploit them when there are no better options and your stack is growing too high.

Tetrises
If your primary objective is to reach the highest score possible, there is no better strategy than to seek to clear four lines at a time (known as a Tetris) as frequently as possible. In order to do this, you must plan out and set up your stack so that you build up nine out of ten columns, leaving one column empty for the arrival of an I piece that will earn you a Tetris. There are several strategies when it comes to choosing the column to leave open, but there are two columns in particular that should be favored over others: the column on the far right, and the fight column from the right.

If you choose one of the far columns, it is better to choose the right column over the left since the I piece (typically) spawns closer to the right side, and so you have less distance to travel to reach it compared to the left. Choosing a far column gives you the greatest flexibility in terms of how you choose to build the stack over the nine remaining columns. You don't necessarily have to plan things out, and if you get in trouble, you can throw other pieces in the far column in order to keep your stack clean. The only drawback to using a far column is that you might have trouble reach the column if your stack gets particularly high, and the pieces are falling very quickly.

If you choose the fifth column from the right (again choosen because the I piece spawns just slightly closer to the right) you will divide your stack into two parts occupying five columns on the left and four on the right. In general, if you can place a T piece on the bottom of the four columns on the right, you will establish a location to distribute all of the S and Z pieces that appear, Z pieces on the left side of the T, and S pieces on the right. You must then distribute any J, L, O and remaining T pieces across the five columns on the left. In a pinch, you can throw a T piece on the right stack, as long as you cover it with another T piece to restore a landing spot for the S or Z blocks that you broke.

T-Spins and other twists
If you accidentally create a gap which looks as if you can't possible squeeze a piece into, it's possible that you might be able to twist a piece into place by rotating it at just the right moment, usually just before it's about to lock in place. This is most easily done with a T piece, and if you do this it's called a T-Spin. Most newer versions of Tetris offer bonus points for T-Spins.

While there are times when the creation of opportunities for T-Spins are purely accidental, many advanced players actively seek to create those opportunities intentionally, in order to exploit them for points or use them to attack other players. To create such an opportunity, you need to carve out space for a T piece (flat side up) and then cover the left or right side of this space. Then you can drop a T piece in on it's side, and rotate it around until it can clear the two lines beneath.

T pieces are not the only pieces capable of twisting into positions buried deeper in a stack. Examine your stack carefully and look for opportunities to twist other pieces into position. They don't have to be perfect fit, and they may not even clear lines, but they may help set you up to clear lines with another piece.

Dropping
Choosing when and how to drop a piece comes with experience. When you have the opportunity to use both soft and hard drops, you should favor hard drops and save soft drops for situations that truly require them. Hard drops can be used when you don't need to slide a piece into position, and it can safely land on top of the stack. Use soft drops whenever you need to first lower a piece into position, and then slide it into a gap in the stack, or to perform a T-Spin. If you use a hard drop in these situations, you will not be given the opportunity to adjust the piece after it lands.

Holding pieces
In versions of Tetris that offer it, the ability to hold and release pieces is an incredibly powerful tool which can help you in almost every conceivable situation. In particular, they are very useful for achieving Tetrises. Use the hold button under these circumstances: When you first start playing Tetris, you may want to make liberal use of the hold feature, trying to ensure that you build the best stack possible and stay alive longer. However, a you get better at playing, you should rely on the hold feature less and less. You should strive for the ability to create stacks that can accommodate every piece so that you don't actually have to rely on the hold feature to survive, saving it for I pieces to be used in Tetrises, or extreme emergencies.
 * If you are not currently holding a piece, press the hold button when the current piece has no good landing position on your stack, but the next piece does.
 * If you are currently holding a piece, consider swapping it out with your current piece when there is a better position on your stack for the hold piece than your current piece.
 * If you are setting up for a Tetris, and an I piece comes along before you have four lines ready to clear, hold the I piece for the moment when you are ready to use it. Don't hold I pieces longer than you have to unless you can see another I piece coming up in the next queue.

It can be used to save you at the last minute if you're about to drop a piece in the wrong place by mistake, but then you risk the chance that the piece you were holding is actually a worse fit than the piece you swapped out for.