Originally Posted by randomwebpage
The no-no interval, or tritone
There's another reason why the G7 moves so satisfyingly to the C. It's called the tritone interval. This interval will make your straight hair curly, your milk go sour, and will propel the G7 smack into the C with cataclysmic force. (That last sentence sounds great if you pretend you're Charlton Heston.)
The tritone is an interval, which means it's two notes: B and F. Play a B and an F, listening to how unusual they sound together and how much tension they produce.