I would like to add that your tension can vary with your mood as well. On a large project, this will tend to even out, but it's good to check your work occasionally if you're making something that has to fit.
All knitters need a variety of needle sizes. By using the size needle that gives you the gauge you need, you avoid the messiness that can occur when you try to knit more loosely, or the struggle involved with trying to knit more tightly. (Either of these tends not to work for very long anyway.)
I like the Options because one pair of tips can be used for large flat projects or smaller knitting in the round. I like to have duplicates, so I can do sleeves and socks using two circular needles, but you can do without using Magic Loop, a method that uses one pair of tips and a fairly long cable to knit small-diameter items (sleeves, socks, hats, etc.) without using dpns, although many people prefer their dpns.
and sometimes you just need lots of needles 'cause you have several projects going at the same time that use the same size needles. (my nightstand drawer is full of knitting needles in case I wake up and decide to cast-on...) anneKat
One other thing that can be involved in gauge is the needle manufacturer. Not all needle sizes are exactly the same. There are some manufactures that will correlate a US 6 to 4.0mm and others to 4.25mm. So you have to watch out for that also.
At least with 1, manufacturers are beginning to size them as 1 and 1.5 and with 2 you will now see 2 and 2.5, but the 6 still seems to have that problem. Even with 10 you will now see 10, 10.5 and 10.75.
I never could figure out why we started to go metric when Jimmy Carter was President and then let it go by the wayside. I mean the road signs had both Imperial and Metric. So did the speedometer on my car.
Thank all of you so much for sharing. Once I started listening to the pod cast I realized there is a lot to learn about knitting and sitting here by myself trying to figure it out is not working. While knitting can be personal sharing and working together are what help us move along in our projects, as you all can attest to. Hearing others in the pod cast discuss issues I had/have with my own work is what has brought me to the community. I look forward to more sharing and learning.
Melissa: Check under the "learn" tab, above. There are many videos and tutorials there. Also, just about any knitting maneuver can be found on YouTube. Some of those are better than others. Look for the ones that are more highly rated. (Some of them are harder to see, and you can't really tell what they're doing. Others have a good background and camera angle.)