when working flat, you can join new yarn at the sides of a piece and weave the ends into the seam. in fact, most knitting books specify that you should try to join new yarn at the end of a row. you can waste a lot of yarn this way. there is nothing more frustrating than knitting to within an inch or two of the end of a row only to discover that you don’t have enough yarn to finish.
when working in the round, you can join yarn at any point. the benefit is that there is very little yarn waste, but you will need to find a way to 