How to identify the modulation of CW QSO?

CW QSO is one of the important way to communicate with hams all around of the world. For Amateur Radio, the frequency and modulation are concerned by us. but which modulation is CW QSO, and how to identify the modulation of CW QSO?

What are the common modulations?

There are three modulations including Amplitude Modulation(AM), Frequency Modulation(FM), Phase Modulation(PM). 

AM: The amplitude of the carrier wave change according to the change rule of the required transmission signal, but the frequency remains unchanged;

FM: Instantaneous frequency of carrier wave change according to change rule of required transmission signal, but the carrier amplitude remains unchanged;

PM: The initial phase of the carrier changes with the baseband digital signal. The Phase Modulation happens with Frequency Modulation , but the changes of the two are different.

Which modulation is CW QSO?

We all know CW QSO is working in a fixed-frequency, so the modulation of CW can't be FM or PM. It's easy to know that the modulation of CW is Amplitude Modulation. but how does the amplitude of CW change? if you are the ham of CW, you'll know there are two states including transmitting(1) or not(0). The modulation is called On-Off Keying(OOK), and it's the simplest form of Amplitude Modulation.