If you’re ever knitting a sweater from the bottom up, there comes a point where you work the armhole shaping and then are told to continue knitting until the armhole measures X inches deep — and I think it’s a measurement a lot of people have trouble with. In the event you’re working the body in one piece and then dividing into front(s) and back at the armholes, you’ll be told to put some part of it (most likely the fronts) on waste yarn while you work the other bits. When you put those held stitches back onto the needles, if you leave the waste yarn in place, you have a clear point from which to measure. But without that, it can be tricky to tell exactly where you’re measuring to, as you attempt to measure straight downward from the top of your knitting to the exact row where the armhole shaping first began. And hard to be sure you’re measuring to the same spot each time you check your progress. One option is to pin a removable marker in that row, but I prefer a mini version of the lifeline. When working the first armhole bind-off row (or later if need be) I’ll just run a short piece of waste yarn through an inch or two of stitches that will line up with the armhole edge. Then all I need to do is measure from that line to the top of my work. You can see here I’ve worked just a little over two inches from my first armhole row, so I’ve got a ways to go.
p.s. Keep in mind the best “waste yarn” is anything non-fuzzy or grippy, so it doesn’t leave any fibers behind when you pull it out. Thin, smooth cotton or dental floss is best.
p.p.s. The knitting pictured is my Spiral-Spun Waistcoat in progress. I decided on 3×1 garter rib instead of the 2×2, thanks to a suggestion from Annri in the comments on my swatch post. Thanks for all your input!