I'm in grade seven and I really need to know what diminished sevenths and double octaves are! Any one! any body! I'll take wierd guesses,shots in the dark and the ramblings of the mind but I NEED TO KNOW! send me your replies quickly for the exam date moves ever closer!

A diminished seventh chord consists of a stack of minor thirds, or a diminished triad with an additional third on top.

The diminished seventh between the chord's root and seventh gives it its name. (It is often called a fully diminished seventh to distinguish it from the half diminished seventh.) Note that this seventh can be respelled as a major sixth, thus changing the implied root of the chord:

Dominant seventh is built on the 5th degree of the scale (the dominant) and consists of the triad plus the 7th note from the root.
So in C the dominant seventh is G B D(the triad) plus F (7 notes up from G). It is F natural because it's the dominant of C and not of G.

Thanks loads but do you play the chord in one go or play like a scale from note to note?

u can do either play it as a chord if its written as a chord or play it as a broken chord as in note bu note its still a diminished as u are using the notes of that chord