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If you want to knit an aran or cable pattern you will need to learn how to use a cable needle. These needles are short needles with points at both ends used to manipulate the order of your stitches.
Straight Cable Needles
Curved Cable Needles
Hook Shaped Cable Needles
There a several different types of cable needles available and all will work reasonable well, so the type of needle you choose is really up to you and your own preferences. I tend to use the straight version if I need to use cable needles (often don't for small cables and I tend to use a spare single pointed needle instead of a cable needle most of the time).
If you want to buy a cable needle or two most come in set of two or three with several different sizes in the set. When you are deciding which needle to use, I recommend one that is close to the diameter of the knitting needles you are using and either the same size or smaller. You definitely do not want it to be larger as the stitching of the previous row can become a little tight as you are manipulating them for your cable pattern and a larger needle will make the task just that much harder.
Details of How to Use a Cable Needle
How you use the needle will change depending on the instructions for the cable you are trying to knit, but basically making cable involves temporarily slipping a few stitches off the left hand needle (assuming you are right handed) and onto the cable needle. Those stitches are either held on the cable needle at the back or the front of the work while you work some stitches on the left hand needle.
After knitting the required number of stitches off the left hand needle you then knit the stitches on the cable needle.
So what you are doing is changing the order in which the stitches are knit as your work your way through your row to 'twist' small groups of stitches to form your cables.
For example on the cable row in the stitch pattern above you would purl to the first cable block of four stitches, slip two stitches onto your cable needle and then take the stitches and the cable needle to the back of the work and hold them out of the way.
You knit two stitches off the left hand needle and then the two stitches on the cable needle which forms the 2 x 2 twist of the cable.