So you're knitting along in a pattern and it tells you to:
"knit in pattern until work measures 10" from CO edge".
You go along your merry way, keeping the stitch pattern even, not increasing or decreasing and work until you whip out your ruler and see that your work now measures the aforementioned 10". So you're ready for the next step, right?
(You knew it couldn't be that easy, right?)
Disclaimer: There are two instances where it really is fine to go by the measurement on the ruler.
1) You're working on something that doesn't need to be an exact size like a scarf, a throw pillow, or a stuffed toy.
2) You're working with a yarn that is the exact same size before and after washing and blocking. I hear such a thing exists, though I can't remember the last time I noticed that my yarn didn't change size at all after blocking.
But back to the main idea here: you do not want to go with the measurement on the ruler because you are working with pre-blocked yarn.
Remember when you made that nice big gauge swatch, washed it and let it dry (in the manner in which you will eventually let this cherished garment dry?) Afterward, you measured the number of stitches in a 4" span, then you measured the number of rows in a 4" span (and you took those measurements in a couple different spots, away from the edges so the stitch sizes weren't distorted. Then you averaged them together to get your best measurements of stitches per inch and rows per inch.) Now is the time to pull those measurements back out and use them to figure out when you actually hit 10".
For example, maybe your pre-blocking yarn gave you 26 x 35 = 4" (or 6.5 stitches per inch and 8.75 rows per inch.) After your swatch was blocked and bone dry, you now measured only 24 x 31 = 4") or 6 stitches per inch (because the knit fabric relaxes) and only 7.75 rows per inch.
So instead of getting your ruler out to measure the 10", if you really want a good fit, you're going to multiply your row gauge (7.75) by 10". So, 7.75 x 10 = 77.5. I always round up (I'd rather something be just a smidge too long than a smidge too short), so I would count off 78 rows and know that at that point, I had 10" of work on my needles and I'd go on to the next step!
It probably doesn't seem like it would make much difference, but if you are knitting a sweater with the gauges in the example above that should end up being 24" long, you'd work 186 rows. Or, if you measured each section with a ruler, you'd end up knitting 210 rows, which, after blocking, would add an extra three inches to your sweater! That's definitely enough length to change the look for the sweater, and, if your yarn wasn't estimated to have a lot of leftover, you may end up running short of yarn before you're finished!