Because it's a blanket, it's w-i-d-e so they probably suggest circs because the work would be too squooshed together on straights. You'd be using the circular needles just as you would straight needles, going back and forth, with no joining (i.e., you're still making a flat piece, not a tube). One nice thing about using circs for something of this size is that the bulk of your work will be resting in your lap rather than on the ends of your needles, making it a easier to manage.
There is no right way to knit; there is no wrong way to knit. So if anybody kindly tells you that what you are doing is "wrong," don't take umbrage; they mean well. Smile submissively, and listen, keeping your disagreement on an entirely mental level. They may be right, in this particular case, and even if not, they may drop off pieces of information which will come in very handy if you file them away carefully in your brain for future reference. ~Elizabeth Zimmerman
Just a note, I'm using 29" circulars (I'm pretty sure that's the length...that's my "at home project" because it's too bulky for carrying back and forth) for 193 stitches for a baby blanket, and it's pretty full. Your straights might not be long enough to hold all those stitches.
I knit that very blanket and I did it on straights. The stitches were pretty smooshed together but other than that, I had no problems with it. Here's how it turned out (it's not as dark as it looks in the pics though).
My vote would be for circulars. I try to use them for everything I can now and my shoulders and back really seem to appreciate it. You don't realize sometimes the weight we are suspending while we work on some projects.
If you want to use straights just because you have them and don't want to go out and buy more needles, you should be fine. You can always start with the straights and change to circs if you think they would be more comfortable.
Personally I find the straights akward, especially the long ones that you would need to use for this, but it's what you make..not the tools, so go have some fun.
I agree with the others, circurlars would be easier. And you need to have LONG straights to fit that many stitches on them and I would suggest getting point protecters (you can use erasers) to keep the stitches on the needle.
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