Diminished Scale

It's a scale that just uses the pattern W-H all the way through, so it ends up with eight notes instead of seven.

There are only two different ones:

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

They're used mostly in jazz. You can use it over a diminished chord or a dominant type chord, but to use it with dominants you have to know your chord spellings cold. For example, over C7, you'd most likely use the C# version instead of the C version, as you get more chord tones in the scale:

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

..not to mention the octatonic parts of it (ive been doing it in school, really interesting)

noteboat, aren't there three of them? since everything reapeats itself a minor third apart you still have the transposition of the scale using a minor or major second.

the missing scale would be C# D E F G G# A# B C# which in this case would be a half/whole diminished scale since it starts with the half step instead of the whole step. this half/whole pattern fits perfectly into dominant chords since you have all the chord tones 1,3,5,b7 plus both a lowered and raised ninth. (common alterations of dominant chords in jazz)

Boy, I really shouldn't post without sleep!

You've actually got two patterns:

C-D-Eb-F-Gb-Ab-A-B-C = WHWHWHWH
C-C#-D#-E-F#-G-A-Bb-C = HWHWHWHW

(the scales I posted, both starting from C)

plus:

C#-D-E-F-G-Ab-Bb-B-C# = HWHWHWHW
C#-D#-E-F#-G-A-Bb-C-C# - WHWHWHWH

The fourth scale is the same as the second one, so three total scales.

You're right about the chord tones - I think I'd showed that, but didn't elaborate.

The HW pattern is sometimes called the 'symmetric diminished' scale by some writers, to distinguish it from the WH pattern, but I really don't see much of a difference... since it's octatonic, you'll have two notes with the same name no matter how you count the pattern, and WH from C is the same as HW from B.