Sandwich
The Sandwich pattern (挟み込み hasamikomi), also known as Key (鍵積み, kagitsumi), is usually learned after a player gets used to chaining. Sandwich is very powerful and versatile and is used by both beginners and experts alike.
Sandwiching (concept)
The idea is to "sandwich" colors between the other colors. The falling Puyo will connect to each other vertically.
2-1-1 Sandwich
The most common form of Sandwich. Very space efficient.
1-2-1 Sandwich
Used every now and then depending on what pieces you get. A littler cumbersome depending on what you're tying to accomplish, but it works.
1-1-2 Sandwich
Similar in structure to 2-1-1 Sandwich, except the group of two is on the top part. You'll have to be a little more careful if you're chaining on the second floor.
Mixed Sandwich
What makes Sandwich powerful is that you can switch between its forms more fluidly than with Stairs forms. You can do this by using 3-0-1 Sandwich and 1-0-3 Sandwich.
3-0-1 Sandwich
Refers to the blue group in the following chainsim.
1-0-3 Sandwich
Refers to the blue group in the following chainsim. You might accidentally cut your chain short if you leave a blue Puyo in column 2, but notice that it's possible to use the blues as an alternate trigger for your chain. This is a useful fact to know for offensive and defensive purposes.
Both 3-0-1 and 1-0-3