Transmission lines are the cables and supporting equipment that carry large amounts of power at high voltages from generating plants, typically located in remote areas, to the cities and towns where the power is consumed. The electric industry defines transmission as any line with a voltage greater than 39 kV, or 39,000 volts, though you will find some companies defining transmission as anything over 50 kV. In these cases, anything between 39 kV and 50 kV is considered sub-transmission.
Common industry voltages include 69, 115, 138, 230, and 345 kV. While there are other transmission voltages, these are the most common. The largest commonly found is about 750 kV. Above 1,000 kV, some transmission lines convert to DC power since AC lines do not function as effectively at high voltages. And so these ultra-high voltage transmission systems carry DC power in some areas of the country. In general, the higher the voltage of the transmission line, the more power it can carry, which of course is the advantage of the higher voltage.
There are a number of different types of towers that carry transmission lines. Standard four-footed towers are often preferred because they are more stable and resistant to winds and weather. If a right-of-way is very narrow the one-footed design may be used, which is typically a concrete or metal pole. The H-frame tower is an older design and is typically a wooden structure.