When to use these: half-dim, sus and slash-chords?

[Nanne]

In my band we only use maj-chords at the moment, so I wanted to find out when we should use other chords. I've read some things about chord progression and chord extension on the internet, but there were still some things unclear to me:
*When can or should you use half-diminished chords?
*What are those sus-chords for? I can't really find a use for them, they just sound weird all the time..
*Are there rules for how you should use those slash chords, or should I just go and try out?

Great site btw

[NoteBoat]

Half-diminished chords, now usually called m7b5 chords, are the four-note harmony based on the seventh note of a major scale. You can use them to step into the root (Bm7b5 -> Cmaj7 for instance) or substitute them for the V chord... a Bm7b5 is B-D-F-A; a G9 is G-B-D-F-A. You're essentially playing the 9th without the root.

Suspended chords usually resolve to the major chord of the same root. It's often a way to delay a chord's resolution - try G7 -> Csus -> C and you'll see the effect (G-B-D-F to C-F-G, holding two chord tones... then to C-E-G, again holding two chord tones - makes a smooth change, but boring if overdone)

Slash chords are simply a way of indicating the bass note progression. Come up with a bass line that works, like C-B-A-G-F-D-G-B... and incoporate it into the chords:

C-C/B-C/A-C/G-F-F/D-G7-G7/B

[Nanne]

Ah, thanks alot, gonna try those when I get home
But on more question about the diminished chords... I see I've been using the 'full' diminished chord where you said I should use the half diminished... So now I'm wondering, where should I use the full diminished chord?

[NoteBoat]

A full diminished chord is composed of all minor thirds.... that leads to the formula 1-b3-b5-bb7 (often written as the enharmonic 6). It's a neat chord - there's only three of them:

C-Eb-G-A
C#-E-G#-Bb
D-F-A-B

the next one in the series is Eb-G-A-C, the same as the first.

You can use it in place of any diminished chord, and it works as a substitution for any 7th - just use the dim7 a half step higher:

B7 = B-D#-F#-A
Cdim7 = C-Eb(D#)-Gb(F#)-A

three of the tones will be the same as the 7th chord, which makes it a really good replacement.

[Nanne]

Thanks alot NoteBoat! You're really helpfull!

[undercat]

You can use it in place of any diminished chord, and it works as a substitution for any 7th - just use the dim7 a half step higher:

Wow that's cool.

[eduardo]

Note Boat, your explanation has been really helpful. Though I knew about these chords and included them in my play, I never saw a better hint on how to use them. It has been quite a discovery and I want to thank you. Please keep posting helping comments

eduardo