Waveguide is the mysterious "pipe in the sky". It has its own terminology set, such as WR-90, flex guide and the like. In this case, Paul has compiled a nice table of names used to define waveguid.
Waveguide was invented in 1932 at Bell Telephone Laboratories.  It is a very low-loss means to convey a microwave signal between antenna and the transmitter-receiver systems. (It has a small fraction of the loss per foot of almost any coax.) The pipe-like waveguide used to adorn the corners of the boxy ATT microwave relay towers scattered every 100 miles or so across the countyside... before optical fiber.
Waveguide may be rectangular or round. (Yes, you really can use 3/4" copper water pipe at 10 GHz... if you're careful!) Rectangular often found easier to work with from the standpoint of propagation mode stability, the standing waves set up within the pipe. Waveguide is sized for the wavelength of operation, so higher frequencies use smaller waveguide.
There is an lower limit ("lower cutoff frequency") at which a signal won't propagate down a waveguide. The lower cutoff is determined by F(Ghz) = c/(2*a), where a is the width of the broad wall (in cm). ;nbsp;For example, WR-90 has a cutoff of 6.56 GHz. To explore this further, consult Microwaves 101 for a practical treatment of the matter.
Various agencies have created their "standard" name for the various waveguide types. Some people find EIA'a WR- nomenclature easier to remember because the type number is the width of the waveguide in hundreths of an inch. For example, the above WR-90 is 0.90" wide (I.D.).
That said, the non-standard standards for waveguide are given in the table below. Even though the table itself has a lower cutoff frequency of 1.12 GHz, waveguide has been used to transport high power UHF signals at 420 MHz. (At that frequency, though, you can no longer stuff a length of it into your pocket; you do the math!)
Here's the table:
|1.12 - 1.70||14||650||6||69, 103||6.50X3.25|
|1.45 - 2.20||18||510||7||-||-|
|1.70 - 2.60||22||430||8||104, 105||4.30X2.15|
|2.20 - 3.30||26||340||9A||112, 113||3.40X1.70|
|2.60 - 3.95||32||284||10||48, 75||2.84X1.34|
|3.30 - 4.90||40||229||11A||- -||2.29X1.145|
|3.95 - 5.85||48||187||12||49, 95||1.872X.872|
|4.90 - 7.05||58||159||13||- -||1.590X.795|
|5.85 - 8.20||70||137||14||50, 106||1.372X.622|
|7.05 - 10.00||84||112||15||51, 68||1.122X.497|
|7.00 - 11.00||-||102||-||- -||-|
|8.20 - 12.40||100||90||16||52, 67||.900X.400|
|10.00 - 15.00||120||75||17||- -||.750X.375|
|12.40 - 18.00||140||62||18||91 - 107||.622X.311|
|15.00 - 22.00||180||51||19||- -||.510X.255|
|18.00 - 26.50||220||42||20||53, 121, 66||.420X.170|
|22.00 - 33.00||260||34||21||- - -||-|
|26.50 - 40.00||320||28||22||- - 96||.280X.140|
|33.00 - 50.00||400||22||23||- 97||.224X.112|
|40.00 - 60.00||500||19||24||- -||.188X.094|
|50.00 - 75.00||620||15||25||- 98||-|
|60.00 - 90.00||740||12||26||- 99||.122X.061|
|75.00 - 110.00||900||10||27||- -||.100X.050|