Ply. It's something I've never really understood. Recently someone told me you should never knit socks in a single. Oops.
I'm optimistic my knowledge of such things will shortly be improving a a result of yesterday's acquisition of Clara Parkes' The Knitter's Book of Yarn. I picked it up in hardcover for just ten bucks at BMV. It has a whole chapter on ply! as well as tons of info on fiber foundation, spin and color. Can't wait to read it cover to cover. It's sure to help my knitting skill. They also had her companion book, The Knitter"s Book of Wool. Perhaps I should get that one as well?