Which larger puzzle-piece set inspired Pajitnov before he chose tetrominoes?

The larger puzzle-piece set that inspired Pajitnov was pentominoes.

Pentominoes are shapes made from five connected squares. Alexey Pajitnov had played with a physical pentomino puzzle and drew on that experience when imagining a computer game about arranging falling geometric pieces. The name Tetris later combined “tetra,” referring to four, with Pajitnov’s favorite sport, tennis.

Pajitnov initially considered using the twelve distinct pentomino shapes, but he found the concept too complicated for the simple computer hardware and display available to him. He reduced the pieces to tetrominoes, shapes made from four connected squares. There are seven one-sided tetromino types commonly used in Tetris, recognizable by names such as I, O, T, S, Z, J, and L.

This distinction explains a frequent misconception: Tetris was inspired by pentomino puzzles, but Tetris pieces are not pentominoes. They are tetrominoes. Triominoes use three squares, hexominoes use six, and dominoes use two, so none of those was the larger set at the heart of Pajitnov’s original inspiration.