I have been looking over some schematics for an industrial Laser. Some resistors which have low values, say 100 Ohms, are labelled 100R. Makes sense. Same goes for 1000 ohms, 1k. again makes sense. But there are a few that have a additional number. For example I see one labelled 5K1. I assume its a 5k ohm resistor but what does the 1 mean? I was looking at another schematic and it has a bunch of resistors but two of them are 3K9. Again whats with the 9?
Most likely the 'K' position indicates the decimal point position.
So, $$ 5K1 \equiv 5.1k\Omega $$ $$ 3K9 \equiv 3.9k\Omega $$
In the same vein as: $$ 3V3 \equiv 3.3V $$
Just a way to save characters/area, and avoids the issue of the decimal point fading out when printing/copying/faxing.