Circular needles are usually used for larger circular knitting, and then, if the piece tapers down so it no longer fits on the circular needle, you can switch double pointed needles. Yes, they are interchangeable in this way; you can switch from one to the other if you want to.
All circular knitting follows the same concept, where you knit around and around. There are numerous ways of doing this. Most people use circular needles for the larger pieces and DPN's (double pointed needles) for the smaller pieces. You can also use circular needles for smaller knitting. Check out the "small diameter" circular knitting methods listed on the Advanced Techniques page for these methods, in addition to using DPN's.
KnittingHelp Queen Bee
"There is a vitality, a life force, an energy, a quickening that is translated through you into action, and because there is only one of you in all of time, this expression is unique. And if you block it, it will never exist through any other medium and it will be lost. The world will not have it. It is not your business to determine how good it is nor how valuable nor how it compares with other expressions. It is your business to keep it yours clearly and directly, to keep the channel open. You do not even have to believe in yourself or your work. You have to keep yourself open and aware to the urges that motivate you. Keep the channel open."
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