F2L notation
Every F2L case needs a name so you can look up its algorithm. This covers how cases relate across the four slots, the BirdF2L code we use, and the numbered schemes you'll see elsewhere.
What is F2L
F2L (First Two Layers) is the step after the cross: you fill four slots - front-right (FR), front-left (FL), back-right (BR), back-left (BL) - with a corner-and-edge pair each. An F2L case is one arrangement of an unsolved pair, and a notation gives it a name so you can find its algorithm.
Mirrors vs rotations
A case list describes each pair once, for the front-right (FR) slot. How you carry it into the other three slots matters: mirroring and rotating don't give the same case.
We mirror: reflect the FR algorithm left↔right (the M mirror) for the left slots, front↔back (the S mirror) for the back slots, or both. Same case - only the hand changes.
Rotating the cube (a y) until another slot
sits at FR also works as moves, but it's a different case: rotation
re-references the pair, and edge orientation is judged relative to the
slot, so a good edge can turn bad.
Rotate Jb into the back-right slot and
you're really solving Mi - a different
case, different algorithm. A mirror doesn't do this, so we always place a case
by mirroring, never rotating.
A case and its mirror are identical to recognise and equally hard (just the other hand), so we treat them as one case, roughly halving the set. Mirroring leaves the diagonal alone - white-on-a-side stays distinct from white-on-front/back - leaving 167 cases.
BirdF2L notation
BirdF2L (by Andrew Robbins) names every case with a two-letter code like Jb. The letters are Speffz sticker positions, so the code says exactly where
the pair sits - read off the cube, no lookup. Our F2L algorithm database uses it.
Speffz letters every sticker A–X (see the Speffz Lettering guide). Corners are UPPERCASE, edges the same letter lowercase.
Reading a code
Cases are shown for the front-right (FR) slot. A code is <Corner><edge>:
- uppercase = the corner's Speffz position, read from its white (D-colour) sticker;
- lowercase = the edge's Speffz position, read from its front-colour sticker.
Speffz says where a piece comes from (the opposite of last-layer
notation). A solved FR pair is Vj.
Jb 
Pp 
Mi Number notation
Most other lists number cases instead. SpeedCubeDB counts F2L 1…F2L 41 (plus AF2L 12a for advanced
ones); other tutorials number their own way. A number tells you nothing
about the case, and two sources rarely agree, so it only
means something next to its own list.
Numbered lists also double-count mirrors: a case and its reflection get two numbers (e.g. SpeedCubeDB's F2L 40 and 41). A BirdF2L code is read off the cube, so it means the same thing everywhere and never splits a case from its mirror.