Which is better in planning a project? To buy yarn by the total number of ounces or by the total number of yards?
If I am knitting something that uses a double strand of yarn, should I seam with a double strand as well?
To answer the first question, I believe you need to buy enough yarn to the amount of yards or metres rather than the ounces as long as you’re buying the same weight (fingering, worsted, chunky etc.). I don’t know the answer to your second question…sorry!
I usually seam with single strand with MOST yarns, even if knitted double strand. If your yarn is lace-weight and not sturdy enough for seaming, then double it.
Depending on the yarn, for some garments it is better to split yarn for seaming. It will make the seam less bulky.
You shouldn’t use a double strand for sewing in most cases. I’d just pick the one that is smoothest and has the most stability. If the yarn is “tender” (if you pull a strand of it from each end and it quite easily comes apart) you may need to use a different yarn altogether than what you knitted with (something that will wash the same way as the item). Most times, when done right, the seam yarn will not show.