The Record Producer
Page 65
Chapter 5
The all encompassing reality of wave relationships.
These two signals will compliment each other. What happens when
they are not congruent is the phenomenon you must train your ears
to detect.

Set up two speakers. Place a chair in mix position (equal distance
from each speaker). Now, listen to some music. Next, take the two
wires that plug into one of your speakers and flip them. Return to
mix position, and play the same music again.

By reversing the polarity in one speaker, you will cause that
speaker's cones to move positively when they should be moving
negatively, and vice-versa. When this happens, the speaker is said
to be 180 degrees out of phase with its partner.

2 sine waves 180 degrees OUT of phase,
2 congruent sine waves,