My most recently completed sweater is both and exception and an example of this rule.
Garter Yoke Cardigan, in last fall's Knit.1 magazine. I've been wanting to knit this sweater all this past winter. I even bought the magazine just to get the pattern, thereby disproving my "yarn first" rule. Even stranger, I've never bought the recommended yarn for the pattern. Actually, I never bought any yarn for the pattern.
What I did buy last year was a bunch of Karabella yarn that was marked down at my lys. I bought some green Aurora 8. I liked it so much that I went back and bought some more in peach, with some Margrite Bulky that I thought complemented it well. It was over 700 yards of yarn, and should have been enough for some type of garment.
It was enough for the Sweet Tee from Interweave Knits. The worry that a bunch of plain stockinette knitting in a light color would visually enlarge my waistline until the possibility of its existence was questioned stayed my needles. *sigh* Consulting a fabulous article on fit in the silver anniversary issue of Vogue Knitting, I learned that a vertical line has a slimming effect. Aha. A cardigan was what I needed. I thought I'd knit the GYC, using the peach yarn as a kind of "muslin" (sewers know what I mean) to see how I'd like to knit the pattern once I actually buy yarn for it.
The knitting didn't take long. I actually finished it in tandem with my last sweater, the green cabled pullover. As you can see, I had to cut it very short. Short sleeves, short cardi. I tried to make up for it with details. The contrasting, picked-up button band was one thing, but the detail I really like is the cuffs.
I copied the cuff idea from a raveler (check my ravelry project for more construction details and a link to the brilliant knitter who thought of deep, buttoned, garter stitch cuffs for this sweater), but I made them short. I had to, I was short of yarn, but I also thought they would be cuter short.
Knitting this "muslin" taught me several things about the pattern. This smooth yarn has too much stitch definition for this pattern, making it difficult to hide the short rows at the back of the neck. I'll look for a fuzzier yarn for my next go-round. Also, I need to keep in mind that garter stitch uses more yarn to cover less area, and buy extra hanks. The pattern shaping is great, though. I probably didn't need those stitches I added under the arm, and thus could knit it again with almost no modifications. The garter stitch yoke really stretches to accomodate my shoulders, making for a very comfortable fit.