The Burmese Position
There are several different leg positions that are possible while seated cross-legged. The first and simplest is the Burmese position, in which the legs are crossed and both feet rest flat on the floor. The knees should also rest on the floor, though sometimes it takes a bit of stretching for the legs to drop that far. After awhile the muscles will loosen up and the knees will begin to drop. To help that happen, sit on the front third of the cushion, shifting your body forward a little bit. By imagining the top of your head pushing upward to the ceiling and by stretching your body that way, get your spine straight—then just let the muscles go soft and relax.
The Half Lotus Position
Another position is the half lotus, where the left foot is placed up onto the right thigh and the right leg is tucked under. This position is slightly asymmetrical and sometimes the upper body needs to compensate in order to keep itself absolutely straight. People who use this position should make a habit of alternating which leg they bring up.
The Full Lotus Position
By far the most stable of all the positions is the full lotus, where each foot is placed up on the opposite thigh. This is perfectly symmetrical and very solid. Stability and efficiency are the important reasons sitting cross-legged on the floor works so well. There is absolutely no esoteric significance to the different positions. What is most important in zazen is what you do with your mind, not what you do with your feet or legs.