In a basic three-stage CW transmitter, what does the key do?

In a basic three-stage continuous wave (CW) transmitter (typically consisting of an oscillator, buffer/driver, and power amplifier), the telegraph key is used to generate Morse code. It does this by interrupting or allowing the transmission of the RF signal, which fundamentally means it is changing the amplitude of the carrier from zero (off) to maximum (on). While we often think of a key as simply an "on/off" switch for the signal, technically speaking, this on/off action is a crude form of Amplitude Modulation (AM). The key controls whether the full amplitude of the carrier wave reaches the antenna. Switching the oscillator directly is usually avoided because it causes a frequency shift known as "chirp." Key clicks and chirps are unwanted side effects that require specific filters or buffer stages to reduce, not the function of the key itself.