The base is the structure directly beneath the empty cells in the pattern. Creating the correct holes in the stack is easier to accomplish when a "sawtooth" or "stairs" structure is maintained in the direction of the diagonal you are creating. By maintaining this structure, holes can be covered with others to finish holes and create the foundation for new ones. S, Z, and T are very useful for creating the sawtooth structure or covering holes, although any piece can be used to create useful structures.

Stairs structure

Variation

Variation

Variation

Situational

O-1

I-1

I-2

J-1

J-2

J-3

L-1

L-2

L-3

S-1

S-2

Z-1

Z-2

T-1

T-2

T-3

T-4

Placement Possibilities

O Tetromino Combinations

I Tetromino Combinations

J Tetromino Combinations