The RST (Readability-Strength-Tone) System

The RST System of Signal Reporting has been used for years (circa 1934) as a shorthand method of reporting Readability, Signal Strength and for CW, Tone (i.e., quality of the CW tone). For voice contacts only the R and S are used. The S component is usually not the same as your S-Meter reading as most S-Meters aren't calibrated 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 practice but a fact of DXing/Contesting

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

Infrequently used is the addition of a letter to the end of the 3 numbers.
These are: X = the signal is rock steady like a crystal controlled signal;
C = the signal is chirpy as the frequency varies slightly with keying;
and K = the signal has key clicks.

X is from the early days of radio when such steady signals were rare.

Today most all signals could be given an X but it is hardly ever used. It is helpful to report a chirpy or clicky signal by using the C or K, e.g. 579C or 579K.

It is very common to send RST reports in abbreviated form, for example 599, is sent as 5NN. "N" in place of the number "9". Also another time saver is for the zero using a long "T". "T" is sent in place of the number zero as in "POWER HR IS 3TT WATTS". There is a number code for all numbers, however,  the N and T codes are the most common ones.
Also CW stations sometimes report their zones as "A4" or "A5" instead of sending "14" or "15".

1 = A,   2 = U,   3 = V,   4 = 4,   5 = E,   6 = 6,   7 = B,   8 = D,   9 = N,   0 = T
 
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