6. Phase
Because a cycle can start from any point on the wave, it is possible to have two waves that have similar shapes that still have different amplitudes at different periods in time. The sine waves previously mentioned have started at zero amplitude at zero degrees reaching a positive maximum at 90 degrees ,falling back to zero at 180 degrees, reaching a negative maximum at 270 degrees and then returning to the start at 360 degrees.
However, it will not always be like this. Combining different types of waves with different phases are often very useful to engineers. For example, if waves which are inversely out of phase are played simultaneously, the result would be destructive interference. This is useful in that it can remove unwanted sounds entirely, but it could also remove the things you do not want to get rid of.
Constructive interference on the other hand occurs when two identical waves with no phase difference are played together. This will double the amplitude but again, this does not mean twice as loud.