What is a "transmission line" speaker?

Thanks again to everyone. Gotta pretty good thread started here and drug in several of the 'Gon faithful. As usual, thisngs are not as simple as they appear.

Twl: Can you point me to a website that has info on the design of your cabinet ? Sounds pretty interesting. Sean
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You should check out the Buggtussel line of speakers. Some very good stuff. The company is owned by Kevin Blair, he is one of the leading people on transmission line design.Their website is www.buggtussel.com

Sean, here is a link with the dimensions that I used. This guy made some for an 8" driver, and then put a cover over the hole, and used some 4" instead. BTW, the "wings" he uses have no resemblance to the "wings" I used. Mine are "swinging doors" on the sides that go from top to bottom.
http://melhuish.org/audio/diy12.htm

These plans are taken from the Lowther Club of Norway design which is no longer posted on their site. Download these dimensions if you are interested, because it's getting hard to find them. These pipes work great with Lowthers.

I use Green Mountain Audio Diamantes that are a quarter wave transmission line. The speakers are cast marble and two pieces each. The upper part contains the speakers and crossover while the bottom/base contains the folded network for the transmission line. The tube is many feet long and filled with long fiber sheeps wool to slow the air flow. I have experimented with slightly changing the damping by altering the length/amount of wool by only 1.5 ft or less makes a very noticeable difference. I had some left over from an update from series 1 to series 2 many years ago which also had a crossover change included. The speakers are rated at about 87db but sound best with a lot of power and cranked up loud like over 90 db average. The bass is very quick and dynamic, no residual warmth, extends to the low 30hz range in my room but very little below that. It is a very different sound than my B&W's.