In English, they're your knees. In Sanskrit, the word for knee is janu. (Think: janu sirsasana). Regardless of how you refer to them, your knees need to be protected-- in life, sports, and yoga. Most of the time, yoga is good for your knees, even those that are compromised by injury and/or overuse (that's you, runners); however, yoga is not always good for your knees. Like anything else, you can do more harm than good if you aren't mindful.
Here are the essential pitfalls to avoid to safeguard your knees while doing yoga:
1.) Stack your joints. Especially in warrior poses and lunges, make certain that your knee does not go past your toe. In other words, stack your knee joint directly atop your ankle joint.
2.) Flex your feet. In pigeon, double pigeon, frog, and other similar poses, flex your feet. This will help protect your knee from bearing all of the pose's emphasis.
3.) Activate the muscles around your knees. In poses where your legs are straight, including forward bends, triangle, and inversions, remember to lift from your quadriceps, the large muscles above your knees (in your thighs).
4.) Never lock. While some poses encourage straight legs (the above mentioned forward bends, triangle, and inversions among them, along with the pose below), it's important not to lock your knees or hyper-extend them. These actions wear down on the joints.
5.) Skip the Bikram classes. In my personal experience, I have known several yogis- men, in particular- who have tweaked or blown out their knees by doing a Bikram-style yoga practice with knees that were vulnerable to injury. As a general rule, it's important to choose a sensible style of yoga for your body. If you're working with an injury, stick to a gentler practice. For a guide to different styles of yoga, click here.