Hey Robroske,

Just to add a litte more to what Jim explained -- the F in a G7 chord is actually called a b7 (Flat 7 or Minor 7). It helps to differentiate between Major 7th (F#) and Minor 7th (F) because they're drastically different sounds. It's not just any 7. :)

Whenever you see it notated like G7 (as opposed to GMaj7 or Gm7) it will always mean a G Major Chord with a b7 (Flat 7) by default. This is also called a Dominant 7th chord.

This is all the long way of saying that there's more than one type of 7.

-Carl.