Reference Card

F2L: The Four Doors

The four basic inserts every F2L case reduces to. Frame: white cross on the bottom, target slot at the front-right — rotate the cube (y/y') to bring any slot there.

The doors

Door	Recognize	Type	Practice setup (from solved)
R U R'	Corner over its slot, white right; edge at the back of the top	Split	R U' R'
F' U' F	Corner over its slot, white front; edge at the left of the top	Split	F' U F
U R U' R'	Made pair along the top's right edge, white front	Made pair	R U R' U'
U' F' U F	Made pair along the top's front edge, white right	Made pair	F' U' F U

Principles

Rule	Why
U is the only free move	After the cross, U disturbs nothing solved. R and F each borrow one cross edge (DR, DF).
Triggers are transactions	The opening R parks the DR cross edge in the empty slot; the closing R' returns it. Balanced turns = safe cross.
Two triggers, one principle	The F-doors mirror the R-doors; the made-pair doors are the split doors with a U-alignment prepended.
Case ≠ door yet?	Use free U moves (and R U R'-family pokes to lift buried pieces to the top) until a door state appears. Systematic pair-making is the next lesson.
Setup = inverse	To manufacture any case from solved, apply the door's inverse. Solving back to full solved is the built-in pass/fail check.

Sources: J Perm F2L · Ruwix F2L case list
Part of the speedcubing course · Course home · Notation · Glossary · Lesson 3