After one has learned to have a firm erect seat, one has to perform, according to certain schools, a practice called the purifying of the nerves. This part has been rejected by some as not belonging to Raja-Yoga, but as so great an authority as the commentator Shankarâchârya advises it, I think fit that it should be mentioned, and I will quote his own directions from his commentary on the Shvetâshvatara Upanishad: "The mind whose dross has been cleared away by Pranayama, becomes fixed in Brahman; therefore Pranayama is declared. First the nerves are to be purified, then comes the power to practice Pranayama. Stopping the right nostril with the thumb, through the left nostril fill in air, according to capacity; then, without any interval, throw the air out through the right nostril, closing the left one. Again inhaling through the right nostril eject through the left, according to capacity; practicing this three or five times at four hours of the day, before dawn, during midday, in the evening, and at midnight, in fifteen days or a month purity of the nerves is attained; then begins Pranayama."