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The transmission line system is a 'direct radiator' alignment that is designed to use the 1/4 wave resonances in a waveguide to enhance and extend its useful range of operation. The waveguide reverses the phase of the driver's rear output by the time it reaches the terminus, thereby reinforcing the output near the lowest resonance frequency of the waveguide. Transmission lines tend to be larger than the other systems, due to the size and length of the line required by the design. The payoff is an extended low end response and a characteristic sound that's appealing to many. The better transmission line designs also take the waveguide's other resonance modes into consideration, as these will otherwise impact the design's useful range of operation.
There is a lot of information about transmission line speakers available on the Internet, but unfortunately a lot of that information is either based on 'classical' transmission line design (which is basically obsolete), and/or is just flat out inaccurate. Fortunately these days there are quite a few software tools (including the freeware Hornresp tool) that can be used to simulate transmission lines, which makes the design process a lot easier.
Note also that the design process for a 'full range' transmission line speaker is likely going to be quite different to the design process for a transmission line subwoofer. That's primarily due to the fact that you will need to suppress resonances in the former by stuffing and/or lining the box because you don't want those resonances coloring the response of the speaker, while in the case of a transmission line subwoofer you would likely try to avoid stuffing it as much as possible because any stuffing in the box will reduce the output at low frequencies. As this is a site for designing subwoofers, it will concentrate on the latter approach.

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Usually, only drivers which have low Qts (0.25 - 0.4) , Qes (0.3 - 0.4) and Fs values are suitable for transmission line systems. However higher-Q drivers can be used. Just ensure that you simulate the results in a proper modeling program first before committing to a build!

Transmission line enclosures are usually a bit more complex to build than your common vented box, and you should spend a lot of time making sure that your simulation is correct before committing it to wood to make sure that you're not disappointed with the results.

There is a variant of the transmission line called the mass-loaded transmission line. This is basically a transmission line that is terminated by a vent. There are several advantages to using this approach, including addressing the pipe resonances along the line and reducing the box size without giving up any noticeable output at low frequencies.

Design Notes:
For these design notes, I will step you through the design process. My tool of choice for designing transmission lines is Hornresp. There may be better tools out there, but Hornresp is simple to use (once you learn how to use it!) and the results are close enough to be useful for most purposes.

Choosing the driver:
For this example, I'm going to use the Dayton Audio PA310. This is a cheap but decent quality 12' pro audio driver with a nice low Fs and Qts, making it suitable for a number of different designs.
The specifications of the PA310 are as follows:
Sd = 530.9 cm^2
Re = 5.2ohms
Fs = 39 Hz
Vas = 99.27 l
Qes = 0.31
Qms = 7.51
Qts = 0.30
Le = 1.4 mH
Xmax = 5 mm
'Classical' transmission line theory suggests that an appropriate transmission line configuration for this driver would be one that is long enough to resonate at 39 Hz, and has a constant cross-sectional area of 530.9 cm^2. But what would the response of such a design be like?

Enter Hornresp, which can be used to simulate such a build.

The following image shows the parameters for a Hornresp sim of this 'classical' type of transmission line. Note: to enter Cms, Mmd, BL and Rms for the driver, simply double-click on the 'Sd' text box - a dialog box will pop up asking you for the t/s parameters for the driver, and Hornresp will use these to calculate the other parameters it uses. Note also that I have selected an 'OD' (offset driver) simulation for this build - the reason for this will be explained later! As for 'Vrc', 'Lrc', 'Ap1', 'Lp', 'Vtc' and 'Atc', I've set those values to zero - we're not going to be using them for this simulation.

A brief outline of what's shown in the sim above:
Ang - for comparison purposes, this shouild always be set at '2.0 x Pi', which basically represents a speaker sitting on the floor
Eg - the voltage that's being applied to the sim, 2.83V
Rg - the resistance of the cable (which I've set to zero for the purpose of this sim)
S1 - the cross-sectional area at the start of the transmission line (in cm^2)
S2 - the cross-sectional area where the driver is located (in cm^2)
Par (1) - the distance between S1 and S2 (in cm)
S3 - the cross-sectional area at the end of the transmission line (in cm^2)
Par (2) - the distance between S2 and S3 (in cm)
So far, so good. At this point, we can select 'Tools..Loudspeaker Wizard', and check what this sim is going to look like

Ok, that looks like what a 'classical' TL would look like, with the driver (the red circle) at one end and the vent at the other. Now, let's see what the predicted response of this sim looks like..

Ugh, that looks pretty bad. The resonance frequency is where we expect it to be, but there's this massive notch in the resonse between 100 Hz and 200 Hz. Why is that? Well, because of resonances along the transmision line. That's because the transmission line does not resonate at one frequency - it resonates at many frequencies. We can see what the result of this looks like by looking at 'Output 1', not the 'Combined' output..

So, we're now looking at the response from the vent alone. See that big peak between 100 Hz and 200 Hz? that's one of the resonance frequences of the straight transmission line, and if look closely you'll see that it's three times the lowest resonance frequency, 39 Hz, which works out to 117 Hz. This resonance is out of phase with the driver's output, which results in the big notch that you see in the 'Combined' response. This is why building a transmission line subwoofer with a constant cross-section with the driver at one end and the vent at the other is so bad - it introduces this huge notch in the response that basically limits the effective passband of the subwoofer.

So, how do we deal with this notch? Well, one effective way is to simply locate the driver partway down the line rather than right at the end. Let's see what happens when we do that..

Transmission Box Parts

So here's what the response looks like with the driver moved to 73.1 cm down the line (now do you see why I chose the 'Offset Driver' type of Hornresp sim?). The notch between 100 Hz and 200 Hz has now been eliminated, and the sim shows a response that extends up to 200 Hz before the next notch appears. Note that there is a peak at 200 Hz, but in reality this peak will not be so high as Hornresp does not include the impact of box losses in its sims.

At this point we could stop here, but I want to refine this build a bit. First of all, a resonance frequency of 39 Hz is a bit too low for this driver (it only has 5mm Xmax,