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 and to slip stitches off either end.
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.
Because these needles are shorter than single pointed needles, the items they are used to make tend to be small and working with them can be quite fiddly.
Most sets come with four needles, you use one as the right-hand needle and the other three hold your stitches, so you have a total of 60cm (3 x 20 cm) to hold your stitches. But as the stitches can slip off either end, it is generally not a good idea to load up the needles with stitches as, unless you are very careful, it is very easy to lose some of them off the needles as you are working.
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!
And I generally avoid double pointed needles and use a pair of circular needles instead as they are easier to manage and you tend to get a much better result that way.
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 a more advanced form of knitting discussed on this site.
© Alne | Flickr - Jumbo Knitting Needles
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.