Running alongside the sushumna nadi, on either side of the spine, are the ida and pingala nadis. Ida refers to the chandra (yin) energies of the moon while pingala refers to the surya (yang) energies of the sun. 
The flow of these two channels is disputed. Modern teachers generally teach that the ida begins in the muladhara at the base of the spine and rises up the left side of the spine until it reaches a chakra. It switches sides at each chakra until it reaches the back of the head. Climbing over the head, it comes down the forehead until it ends in the left nostril. The pingala runs similarly but begins on the right side and ends in the right nostril. Together they form a caduceus, two snakes spiraling their way around the sushumna nadi.
Dr. Motoyama’s research reveals that none of the yogic texts actually describe in detail the paths of the ida and pingala. There is certainly no discussion of the nadis crossing at the chakras. Implied is that the nadis flow up alongside the spine much like the Urinary Bladder lines in Chinese medicine. His experience shows that these channels pass through the nostrils on their way to their termination in the ajna chakra (at the point between the eyebrows.)
An interesting thing happens to the flow of energy in our ida and pingala channels: about once every ninety minutes or so, our breath switches sides. See if you can tell which nostril is more open right now. Generally if you close one nostril while you breathe, and then the other, you can tell which one is more open. When we are healthy, the breath switches nostrils regularly, every ninety minutes or so. When we are ill, the time between switching is longer, maybe every few hours. It has been said that when death is near, the breath does not switch nostrils at all; we will all get to test out that theory one day.
When the breath is flowing out of the surya (the solar or right) nostril, we are in a yang, energized state. When the breath is flowing out of the chandra (the moon or left) nostril, we are in a yin, passive state. There are several forms of pranayama that help to balance the surya and chandra energies such as nadi shodana (an exercise designed to cleanse the nadis). 
According to many teachers, there are certain activities that must be abstained from if the wrong nostril is open. For example, Pattabhi Jois, in the book Yoga Mala, warns that one must not make love when the sun is shining, or when the right nostril is open. When the right nostril is open, it is the same as the sun shining. Most students do not think to check their breath before commencing lovemaking; however, if you do decide to check and you notice that conditions are not optimal, don’t despair. There is a way to change the flow of the breath so you won’t have to tell your anxious lover to wait for a couple of hours. A sinus reflex can be stimulated, allowing the breath to switch sides within a few minutes. There are a couple of ways to tap into this reflex. One way is to lie on your side that is already open with that arm extended over your head and used as a pillow. Another approach is to sit and shift your weight to the buttock of the open nostril. However if neither intervention works, please do not blame yoga for your lover’s frustration.
As with all forms of pranayama, or controlling of energy, caution is advised. Many students who have managed to raise their kundalini energy without properly cleansing and preparing their bodies have suffered severe physical and psychological damage. These techniques should be practiced only under the guidance of a teacher who has been there before and knows the way.
- — The two words that make up the word “Hatha” in Hatha Yoga are ha and tha. Most teachers interpret ha to mean the sun and tha to mean the moon. Thus the right nostril, being the solar channel, is the ha channel and the left nostril, being the lunar channel, is the tha channel. However, as usual in the world of yoga, even here there is no unanimity. T.K.V. Desikachar in his book The Heart of Yoga defines ha to be the moon and tha to be the sun. But even he admits the left nostril is the lunar channel.
- — Which, curiously enough, looks a lot like the old logo of the American Medical Association, which the AMA borrowed from the caduceus of the Greek god Hermes.
- — These pranayamas will be described in Chapter 15: Moving Energy.