The Guitar Chair replica
The "Guitar Chair" first came to my attention on the news group rec.music.classical.guitar.  The chair is made specifically for the classical guitar player, is sold over the internet on the web site www.theguitarchair.com.  Having suffered from an aching back over the last couple of years and weekness in my legs, as well as being frustrated over having to use different chairs wherever I play, I decided to replicate the guitar chair using the original as a template, making changes that were appropriate for my needs.
The Back frame is reasonably simple.  I used spalted maple.
The swinging middle leg is light and uses a basic joint that is strong.
The middle leg and the seat are attached with piano hinge, again very strong.
This is a better view showing the piano hinge and the middle leg holding device. 
The chair folded and ready for ?????
Does the Chair work?  Mine does!  I think the design is ingenius.  The back rest is totally functional. There is no pressure on the right leg and I don't sit on half a chair.  The darn thing really works.  in building the chair, I made sure that the height was correct for me.  I simply don't know if the "guitar chair's" one size fits all actually will fit everone, but that's somthing you can discuss with the "guitar chair folks" for more information on posture visit "The human Gutarist".

The chair is simple to make, for those that have basic wood working skills & tools. The cost is relative to the type of material you use and what you want the finished product to look like. Because I intend on using mine for performance as well as at home, I had the seat and back rest professionally covered.  If you want further information on building your own, email me.