As the name implies, ladders look like steps to be climbed and occur if one is not careful when knitting in the round. Although they may look like dropped stitches, actually they are formed by the stitches being loose when changing from one needle to the next. They can occur when using four needles or a circular needle when knitting in the round but are more common when using four. Let's look at both types of needles and how to avoid ladders with each.
Four double pointed needles
Knitting with four double pointed needles can be daunting. The problem with ladders arises when you change from one needle to the next. It is caused by the yarn being too loose when you change needles. The three needles with the stitches form a rigid triangle which makes it harder to knit the first and last stitches causing a tension problem. There are several ways this can be avoided.
1) An easy solution to this problem is to give a tug to your yarn when you knit the first and second stitches of the next needle.
2) Some knitters use a set of five needles instead of four to keep the tension more even. With this method you divide your stitches onto four needles and use the fifth as a working needle. In this way there is not as much tension on the stitches because the angle of work is not as shallow.
3) Another method to prevent ladders from forming in your knitting is to knit a couple of stitches forward from the next needle onto the one you've just finished with. This shifts the point of tension so that ladders cannot form.
Circular needles are made of two hard tips connected by a flexible plastic or nylon cord. Most are made with metal tips, but some are made of plastic or bamboo.
Ladders are more common when using double pointed needles but can occur with circular needles also. One reason for the ladder developing is the use of stitch markers. In knitting in the round it is important to use a marker to delineate the beginning of the round. There are a couple of ways to solve this problem.
1) Like with double pointed needles, pull the yarn a little tighter when you come to the marker.
2) A simpler and cheaper solution is to use yarn scraps as markers. Just tie a piece of yarn onto the needle where you want to place a marker. The yarn does not have to stretch as far over a scrap of yarn as it does around a plastic marker. Slide the yarn scrap from one needle to the other just as you would an ordinary marker. Be sure to keep the yarn scrap free so that it doesn't become entangled in the garment.
Ladders are particularly aggravating for knitters beginning to use double pointed needles. And it may be hard to conquer them by changing your tension. Tension can be a problem in itself for beginning knitters. The solution to this is to keep knitting. The more you knit the better you become. Soon you will find that you do not have ladders as a problem to avoid.