I was sitting here, trying to write a simple drop shoulder sweater pattern for about 11 sizes (patternmakers deserve respect for the sheer drudgery of tasks like that, even if the pattern, like this one, is relatively simple) when I had a thought. A HUGE thought. We all know that blocking changes gauge, right? Sometimes quite a bit! And we don't block while we're knitting, do we? Not in the middle of, say, a sleeve, anyway. Right? So WHY does almost EVERY printed pattern say things like "work until length measures 8 inches", "work until piece measures 15 inches", "work for 3 inches"? Do you see what I'm getting at? Those measurements mean NOTHING before blocking! The length that's supposed to be 8 inches on the finished garment could be 9 inches after blocking.
Knitting patterns tell you how many stitches to cast on, both for pattern stitch purposes AND for size; they don't say, "Cast on 14 inches." Similarly, knitting patterns should give length in ROWS, not inches (or centimeters), because that's the unit from a gauge swatch that remains constant, before AND after blocking.
Edited to add: I should elaborate on that last sentence. I don't mean the numbers of rows per inch remains the same, because the whole point of this little tirade is that it often doesn't. What I meant will hopefully be clarified by the following story:
THE BLOCKING QUOTIENT
Say you make a 20-stitch wide, 24-row high swatch which after blocking is exactly 4 inches square (and yes, okay, you really should have made it a little bigger), but before blocking was 3.5 inches square (a not unreasonable change): well, you have the same number of rows and stitches, but your gauge has changed from 5.7 sts and 6.9 rows per inch to 5 sts and 6 rows per inch.
Being a meticulous person, you choose your size based on your after-blocking gauge, and you knit a simple tunic that's 24 inches long and 35 inches around.
You're nervous because it's supposed to be 40 inches around, but the directions said to cast on 200 stitches and you did, so you're hoping for the best. You block it, and--surprise! It's now 40 inches around--huzzah!!--but at 27.4 inches long, instead of barely covering your butt, it's hanging at mid-thigh.
"WTF!?" you say. "I followed the instructions to the letter!" YES, YOU DID! The instructions were wrong, not you; instead of "Bind off at 24 inches," they should have said "Bind off at row 144" (which would have been at about 21 inches, as knitted).
OR they could have said, "Divide your after-blocking gauge by your before-blocking gauge; this is your Blocking Quotient. Multiply any lengths specified in the directions by your BQ to get the length you should knit."
(In the example above, the row BQ would be 6/6.9, or .87.)