Happens when you solve a 4x4 like a 3x3 and end up with one flipped edge pair or two swapped edge pairs. A 3x3 never gets these cases, so they need their own algorithms.
Why parity happens
A 4x4 has no single edge pieces. Each edge is two pieces paired together.
When you pair them, they can end up in a way a 3x3 can never reach. On the last layer this shows up as one flipped edge pair, or two edge pairs that have swapped places. You did nothing wrong; you just need one extra algorithm.
When to use this lesson
- : while making the yellow cross, 1 or 3 yellow edges face up. Put a pair that is not yellow on top at the front, do OLL parity, then carry on like a 3x3.
- : at the very end, two edge pairs are swapped and Ua or Ub cannot fix them.
- Two pairs (front and back): do PLL parity and you are done.
- Two pairs: do PLL parity once, held any way. Three edges are then out of place; finish them as in Place the yellow edges.
Order
Each parity shows up in about half of all solves. Do OLL parity before solving the top layer, because it mixes up the top pieces. PLL parity keeps every other piece, so do it last.
If the top shows a case the 3x3 never has, such as only two corners swapped, that is a sign of PLL parity.
Memory tips
The OLL parity algorithm has 15 moves. Split it into 4 beats:
r2 B2 U2l U2 r' U2 r U2F2 r F2 l'B2 r2
Most moves are U2, F2 and B2, so you rarely need to regrip. PLL parity is much shorter: three pairs of moves, each starting with r2 (r2 U2, r2 Uw2, r2 u2).
Common mistakes
- Turning
r as two layers. Here r is only. Books that write TR mean the same move.
- Holding the cube the wrong way. The flipped pair must be on top, at the front. Two swapped pairs must be at the front and back.
- Stopping halfway to check. Pieces look scrambled in the middle; finish the whole algorithm.
- Doing PLL parity twice in a row. Do it once; what is left is a normal 3x3 case.
Next lesson
Once parity is fixed, you have solved the 4x4. Practice all four lessons to get faster.