The cross is a concept first proposed in the early 80's by David Singmaster, and is the first step for most layer by layer methods on 3x3x3. It involves solving the four edges of one layer into place all at once. Beginner methods will often start by teaching the solver to place one piece at a time, but a more advanced cuber will be able to determine during inspection how to place all four edges at once. Forming the entire cross can always be done in 8 or fewer moves. In some cases cubers will start a solve by building an extended cross, which involves a cross as well as one or more corner-edge pairs.
Most beginners solve the cross on top, then once complete will rotate the cube so that the cross is now on the bottom. Then they will proceed to do F2L. Although you can solve the cross on any side, including the front, back, and the right or left sides, most cubers advise doing the cross on the bottom, as this provides greater visibilty and lookahead for F2L, and also eliminates the need to do the cube rotation to position the cross at the bottom.
- Cross Study - optimal move count stats for solving the cross.
- Rubik's Cube: Cross (Advanced) - Youtube video on solving the cross recommended by Feliks Zemdegs on Twitter (2012/11/26)
- Just Another Rubik's Cube Solver - Web-based solver that will give the optimum solutions for the cross.