If you look at the standard 3x4 phone number grid, each row or column had a specific frequency assigned to it. A single button would play both the row and column tones simultaneously.
edit: I just remembered, it's actually 4x4. There is a fourth column labelled ABCD. Forget what it's used for. I was mostly a theoretical phreak :)
The 6 extra keys (*, #, A, B, C, and D) where meant to be used for remote control of devices attached to the telephone network. They were heavily used for test equipment. Another use was in the old Army AUTOVON system where the letters were mapped to call precedence levels.
If you look at the standard 3x4 phone number grid, each row or column had a specific frequency assigned to it. A single button would play both the row and column tones simultaneously.
edit: I just remembered, it's actually 4x4. There is a fourth column labelled ABCD. Forget what it's used for. I was mostly a theoretical phreak :)