The RST code, where the acronym stands for Readability-Strength-Tone, is used by amateur radio operators, shortwave listeners, and other radio hobbyists to exchange information about the quality of a radio signal being received.
| Readability (R) | Signal strength (S) | Tone (T) |
| 1. Unreadable | 1. Faint signal, barely perceptible | 1. Sixty cycle a.c or less, very rough and broad |
| 2. Barely readable, occasional words distinguishable | 2. Very weak | 2. Very rough a.c., very harsh and broad |
| 3. Readable with considerable difficulty | 3. Weak | 3. Rough a.c. tone, rectified but not filtered |
| 4. Readable with practically no difficulty | 4. Fair | 4. Rough note, some trace of filtering |
| 5. Perfectly readable | 5. Fairly good | 5. Filtered rectified a.c. but strongly ripple-modulated |
| 6. Good | 6. Filtered tone, definite trace of ripple modulation | |
| 7. Moderately strong | 7. Near pure tone, trace of ripple modulation | |
| 8. Strong | 8. Near perfect tone, slight trace of modulation | |
| 9. Very strong signals | 9. Perfect tone, no trace of ripple or modulation of any kind |