RST

R: Readability

1 -- Unreadable

2 -- Barely readable, occasional words distinguishable

3 -- Readable with considerable difficulty

4 -- Readable with practically no difficulty

5 -- Perfectly readable

S: 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

T: 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

Radio/CW