When choosing neckline depth, you have three "orientation" choices:
- Below shoulders -- the neckline shaping will begin X"/cm measured down from the shoulders
- Above armhole -- the neckline shaping will begin X"/cm after the armhole shaping begins
- Below armhole -- the neckline shaping will begin X"/cm before the armhole shaping begins
We find the latter two helpful for specifying deeper necklines, especially since armhole depth varies from knitter to knitter. While you may be used to seeing necklines specified the traditional "below the shoulder" way, the latter two methods can be very handy as you design your own sweater.
If you're not sure how deep a neckline to choose, try this article.