Double pointed knitting needles are used to knit seamless tubes of knitting for articles like socks, or mittens.
Basically, as their name suggests, these knitting needles have tips or points at both ends of the needle which enables the knitter to knit from either end of the needle.
In Australia, these needles only come in a couple of different lengths.
But if I buy double pointed needles from overseas I can get them in 13, 20 and 23cm lengths.
Most of my Australian needles are around 20cm in length.
Double pointed needles tend to be available on in the more commonly used standard sizes, from 2.0mm to 5.0mm.
It is possible to get larger sizes but these days you have to really look hard for them!
To be quite honest, I don't know why you would want to use larger double pointed needles.
I have enough trouble managing four or five normal sized needles at a time without making it even more difficult by upsizing!
As indicated above, double pointed needles are used to produce seamless tubes of knitting.
Basically you just keep knitting around and around in a circle.
This knitting technique is called knitting in the round and is one of the more advanced forms of knitting discussed on this site.
Some of the items you might use these needles for are socks, mittens or gloves, very small garments (for example knitted singlets for a premature baby) or to knit the centre of a circular baby shawl.
For larger items, like the body of an adult garment or as your baby shawl grows to the point where your stitches are too bunched up on your double pointed needles, you might want to consider using a circular needle instead.