|They're called carpet cutter blades|
Also, if you have trouble cutting straight with a rotary cutter (and I did, even though that is pretty much the whole point of them), I recommend getting one with a handle like this.
|NAYY*, just a happy user|
This one is from Fiskars and my mom gave it to me. The handle makes it really easy to get a balanced grip, which makes it so much easier for me to stay alongside my ruler.
*In case you are wondering, I don't get money to say nice things. I just wanted to pass along what has worked for me and made me happy. If Janome (the sewing machine manufacturers) ever wanted to give me free stuff to endorse them, I would totally do it. Otherwise, I'm just a girl who loves to sew, no commercial strings attached:)