The RST System Explained

The RST System of signal reporting has been in use for years as a method of reporting.
Readability, Signal Strength & Tone 

The S component is usually the same as your S-Meter reading though most S-Meters aren't calibrated correctly to track the RST System. The RST is also reported on QSL Cards and must be filled in correctly -- e.g., a 569 report for a Voice Contact is invalid. Note that many DX operations and contest stations merely report 59(9) as a convenience to avoid having to log each of the real reports. A questionable practise but a fact of DXing / Contesting.

RST SYSTEM Guidelines

Readability:

Signal Strength:

Tone:
(for Morse code contacts only)

1 - Unreadable.

1 - FAINT Signals, barely perceptible.

1 - Sixty Hz AC or less, very rough and broad.

2 - Barely readable, occasional words heard.

2 - VERY WEAK signals.

2 - Very rough AC, very harsh and broad.

3 - Readable with great difficulty.

3 - WEAK signals.

3 - Rough AC tone, rectified but not filtered.

4 - Readable with practically no difficulty

4 - FAIR signals.

4 - Rough note, some trace of filtering.

5 - Perfectly readable

5 - FAIRLY GOOD signals.

5 - Strong ripple modulated.

6 - GOOD signals.

6 - Ripple modulated tone.

7 - MODERATELY STRONG signals.

7 - Trace of ripple on tone

8 - STRONG signals.

8 - Slight trace of ripple on tone.

9 - VERY STRONG signals.

9 - Perfect tone.