Edge Orientation and Approach to Last Layer
One of my favorite parts about the Petrus method for cubes is the way
the it orients the edges before finishing the F2L. Once I realized that
the FFF approach left two
faces unsolved instead of just one, I started orienting the edges first
and completing up to the LL after, just like in Petrus method.
For those who aren't familiar with Petrus allow me to explain. Suppose
the last two unsolved faces are U and F. If an edge can be correctly
put into position using only U and F turns, it is correctly oriented.
If it is flipped when it is put into position, then it is incorrectly
oriented. The edges will not change their orientation unless another
non-parallel face is used, like R or L.
Edges can be flipped in pairs using sequences like R U R', where R
would correct one flipped edge, U would put another flipped edge into
it's position, and R' would correct the second flipped edge. Solve a
2x2x3 block on a cube and try it out, or check out Lar's site for a
more detailed explanation.
In the picture below on the left, correct edges have a white dot and
incorrect edges have a red dot. In the picture on the right all the
edges are correct. Note that they don't need to be lined up with their
respective faces, all that matters is that they could be aligned using
on U and F.
The first picture isn't that great, so let me say the the bottommost
left edge is blue-yellow and the topmost left edge is blue-orange. The
number of flipped edges will always be even, so anywhere from zero to
eight. I actually had to use that to figure out what the blue-orange
edge was, because I didn't remember after I took the picture.
Similarly, this can save you a few looks while speedsolving.
Now that all the edges are oriented, solve the non-gray pieces using only turns of the U and F faces!
This is very important, because if other turns are used edges will be
messed up, and you'll have to use a 4-look style LL, and that's no fun.
Using only the U and F faces may seem restrictive if you aren't used to
it, but after a while it will be easier because there's fewer different
cases and sequences of U and F are much easier to execute quickly.
First Four Faces - Edge Orientation and Approach to Last Layer - Orientation of Last Layer Corners
