Sometimes I see references to "R" resistors. For example:
Obviously the 100 refers to 100 Ohms. What does 100R mean?
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The idea is that the multiplier replaces the decimal point. This dates back to pre-CAD schematics which were hand drawn and then photocopied and reduced. A decimal point could easily get lost during the copying process. By writing 4k7 rather than 4.7k the risk of these errors was greatly reduced. R was used for a multiplyer of 1 because omega could easily be mistaken for a 0. So ... 4R7, 47R, 470R, 4k7, 47k, 470k, 4M7, 47M.
The same approach is used with capacitors: 2p2, 22p, 220p, 2n2, 22n, 220n, 2u2, 22u, 220u. In the old days larger values were still marked µF so the next decade was marked 2200u but with large capacitor values common now we're seeing 2m2, 22m, etc. I've never seen an equivalent of the 'R' as in 2C2 for a 2.2 F - yet! 2F2 may be more sensible. The current use of 'R' would then be excused (4R7 instead of 4Ω7) on the basis that Ω isn't readily available on most keyboards.
This system may be more popular in Europe.
Thanks to @JasonC for pointing out that the 'R' notation is covered by British Standard BS 1852.
It is quite common to see the letter "R" used as a decimal point. As in 47R9 = 47.9 ohms. And likewise, it is common to see the letter "K" or "M". For example 6K81 would be 6,810 ohms and 2M3 would be 2,300,000 ohms.
Adding to the other answers, sometimes you may even see E used in place of R. So a 100 ohm resistor would be 100E and a 9.1 ohm resistor would be 9E1 for example.
Typically, resistor "multipliers" are represented as:
KΩ (thousands of ohms),
MΩ (millions of ohms),
GΩ (thousands of millions of ohms) ...etc.
Since the context usually makes it clear that we're talking about resistor values, it's common short-hand to drop the 'Ω' so that, for example, you can write "39K"* instead of "39KΩ". But, dropping the "Ω" leaves the problem of how to represent a resistor value when the multiplier is 1. So it was decided that "R" would represent a "x1" multiplier. So now you can write "39R" instead of "39Ω".
The multipliers (R, K, M, G... etc) can also be used as shorthand for decimal points.
So, for example, instead of having to write "2.2Ω", you can simply write "2R2". All the multipliers can be used in this way. A final example: "3.3KΩ" can be written as "3K3"
Note that it is common practice to capitalise the "K" multiplier when referring to resistor values. Technically this is incorrect, as "k" is the official '1000' prefix. But it's just a shorthand, limited in its use to resistor values, and the capital K is in common use in this context.
The notation to state a resistor's value in a circuit diagram varies. The European notation BS 1852 avoids using a decimal separator, and replaces the decimal separator with the SI prefix symbol for the particular value. For example, 8k2 in a circuit diagram indicates a resistor value of 8.2 kΩ. Additional zeros imply tighter tolerance, for example 15M0. When the value can be expressed without the need for an SI prefix, an "R" is used instead of the decimal separator. For example, 1R2 indicates 1.2 Ω, and 18R indicates 18 Ω. The use of a SI prefix symbol or the letter "R" circumvents the problem that decimal separators tend to "disappear" when photocopying a printed circuit diagram.
Also, I've seen, 1. just-like R, also E is being used such as 4E7 etc. 2. the zero for tighter-tolerance sometimes not given, such as 47K, 56K etc.