written usage for resistance values
of electronic resistors

Because it is very easy to miss decimal points (or decimal commas) in parts lists, a convention for writing resistances has developed. By this convention, the decimal separator is replaced by a letter based on the multiplier, for example, “K” (for thousand) or “M” (for million). So, for example

47,000 = 47K  
4,700 = 4.7K = 4K7
4,700,000 = 4.7M = 4M7

Where the multiplier is 1 the letter “R” is used. The symbol Ω was not used because it is not available on many printers. For example:

0.68 = R68 (multiplier is 0.01, silver)
6.8 = 6R8 (multiplier is 0.1, gold)
68 = 68R (multiplier is 1, black)

Later additions were “G” (from the SI's “giga-”), and “T” (from the SI's “tera-”). Notice the code is all in uppercase (a different set of characters, in lowercase is used with capacitors).

Finally, most recently, an uppercase “L” was chosen to represent the submultiplier 1/1000th (SI's “milli-”). For example, 6L8 Ω is 6.8 milliohms.

standards

IEC 60062:2016. Marking codes for resistors and capacitors.
IEC 60062:2016/COR1:2016. CORRIGENDUM 1.
IEC 60062:2016/AMD1:2019. Amendment 1 - Marking codes for resistors and capacitors.

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