When sending CW, what does a “C” mean when added to the RST report?

The RST system stands for Readability, Strength, and Tone, and is used to report signal quality in CW and digital modes. Occasionally, a suffix is added to the numerical report to describe specific anomalies in the received signal. The letter 'C' appended to an RST report, such as '599C', specifically indicates that the signal has a 'chirp.' A chirp is an undesirable, slight shift in the transmitter's frequency that occurs at the exact moment the Morse key is pressed or released, resulting in an unstable, wavering tone. Distractors often play on the letter C. For example, a learner might guess it stands for 'Copy' (as in 100 percent copy) or confuse it with 'K', which is the suffix used to report key clicks.