The period of the wave is this explanation: A group will 'modulate' the wave amplitude; it's like another long wave on top. So the first wave in a group is tiny, the next one is bigger and so on until you get the biggest one in the middle of the group. Then they get smaller again. The last one is tiny, so the biggest wave in the group is in the middle, and if there are 14 waves in a group, the seventh wave is the biggest.
