From Hard Drop Tetris Wiki
Fractals (sometimes also called Spider) are also known as TSD stacking. This is one of the most common T-Spin Methods used in multiplayer. Be wary that it can be difficult to build these without using T-blocks in the stack, therefore presenting significant risk in the event that you don't get enough T-blocks in time for the execution. This pattern can be used to make a stack indefinitely high, but setups stacked three or higher are rare.
Japanese people use the term "Double Dagger" (Double Decker) instead of Fractal.
Fractal Shapes
Fractal 1
Two or more T-Spin Doubles layered above one another in the following even shape.
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Fractal 2
Fractal 2 is a similar idea to Fractal 1, however the shape is uneven. Notice that with F2, the hang must be on the correct side to avoid blocking the hole.
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Fractal Cave
An STMB Cave over an incomplete T-Spin Double. This situation is not uncommon after the T-Spin Triple in Triple Double.
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Two T-Spin Doubles
By changing the position of the overhang, you can change from fractal to an Imperial Cross.
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See also
Back-to-Back T-spin Triple - Essentially the same concept, but with a T-spin triple stack instead. Black Cross - A similar way of building T-spin doubles