And I bet it was fun too, eh?

It's not called "a fifth," because the power chord it is actually considered a chord in theory. The fifth is the dominant note of the root, so with distortion and played together, there wasn't much need for the third (to make a triad) because it sounded good.

The third is a different story. The third is what gives a chord it's color--major or minor; so when you play a third instead of a power chord--root/fifth--you get an incomplete triad because it excluded the fifth.

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