How does teleprompter work?

A teleprompter is simply a computer, a monitor with an inverted/mirrored image, and a piece of glass.

The text the person is to read for their speech is programmed into the computer ahead of time and is "played back" by scrolling up the screen (like credits at the end of a movie).

If you looked at the computer monitor directly however, you would notice:

a) That it is laying down, face-up

b) All of the text is mirror-imaged (backwards)

A sheet of glass is then placed above the monitor at a 45 degree angle, reflecting the image to the speaker. Since the reflection of the glass "mirrors" (inverts) the image, it makes it readable.

The speed of playback is usually very carefully controlled by a human who adjusts the vertical scroll rate to keep pace with the speaker. For example, if he or she were to cough, causing a pause, you would not want the text to scroll off the top of the screen and be out of sync with what they were just saying.

Typically there are three teleprompters used; left, center, and right. This gives the appearance that the speaker is looking around the crowd, when in fact he or she is just picking another teleprompter to look at (and look personable to the crowd).