Half Turn Metric
The half turn metric (HTM), also known as the face turn metric (FTM) is a metric for the 3x3x3 where any turn of any face, by any angle, counts as 1 turn; thus it is different from the quarter turn metric because half turns count as one move instead of two. It is important to note that in HTM a slice move actually counts as two turns, since the centers are assumed to be fixed. Cube rotations do not count as turns, so a 'double layer' turn such as r would count as one turn.
Thus the following algorithm would count as 11 turns HTM: r2 U R' U' M U R U' R' r' (an edge 3-cycle on the front face).
God's Number in Half Turn Metric is 20 moves.