RST Explained

Readability 1 Unreadable 2 Barely readable, occasional words distinguishable. 3 Readable with considerable difficulty. 4 Readable with practically no difficulty. 5 Perfectly readable.

Signal Strength 1 Faint signals, barely perceptible. 2 Very weak signals. 3 Weak signals. 4 Fair signals. 5 Fairly good signals. 6 Good signals. 7 Moderately strong signals. 8 Strong signals. 9 Extremely strong signals.

Tone 1 Sixty cycle a.c or less, very rough and broad. 2 Very rough a.c., very harsh and broad. 3 Rough a.c. tone, rectified but not filtered. 4 Rough note, some trace of filtering. 5 Filtered rectified a.c. but strongly ripple-modulated. 6 Filtered tone, definite trace of ripple modulation. 7 Near pure tone, trace of ripple modulation. 8 Near perfect tone, slight trace of modulation. 9 Perfect tone, no trace of ripple or modulation of any kind.

If the signal has the characteristic steadiness of crystal control, add the letter X to the RST report. If there is a chirp, the letter C may be added to so indicate. Similarly for a click, add K. The above reporting system is used on both cw and voice, leaving out the “tone” report on voice.

Source: ARRL