 # Project Cube Solver #04 – Faces turn metrics

After we created a scrambles generator, our next step is to create the classes responsible for measuring the length of scrambles. There are mainly two metrics:

• Quarter Turn Metric (QTM)
• Half Turn Metric (HTM)

Half Turn Metric (HTM) – also called Face Turn Metric (FTM), is a metric for 3×3 cubes, where every movement of any face is counted as one move. So no matter whether R or R’ or R2 is executed, each one is counted as one move. There is one exception for middle layer moves such as M, M ‘and M2, which are counted as two moves.

Quarter Turn Metric (QTM) – in this metric every 90 degree turn is counted as one move. Thus, for example, the motions R and R’ are counted as one, but the motion R2 is counted as two moves. What’s important, the 90 degree movement of middle layer is counted as two moves, similar to HTM metric. But the M2 motion is four moves.

Moving in the middle layers is counted in this way, because in order to execute them it is necessary to move the outside faces. Luckily, we will not use the center faces because we can use only the outside faces to reach each cube setting.

Here is an example algorithm with the number of moves calculated by both of these metrics:
``` R' F R' B2 U B2 L' D' R B2 D R F' R2 D F D2 B D' F L B D' L' D - 25 HTM, 30 QTM ```

In next article we will focus on creating classes responsible for measuring length of scramble.