Asana is the third rung in the ladder of the practice of yoga.If the yamas and niyamas are the foundation of yoga, asana may be regarded as its threshold. 'Asana', literally, means a seat.Here 'seat' does not mean a cushion or some such thing that is spread on the ground. Asana is a pose of the body or the posture which it assumes at the commencement of the practice.It is called a 'seat', because it is a posture of sitting and not standing. While there exist many asanas, such as the 'sirsha', etc., there is only one set of postures which can be taken as aids in meditation.A sitting posture is asana, because to stand and meditate may lead to a falling down of the body, and lying down may induce sleep. The sitting posture is therefore the most conducive to concentration of mind.That there are many other asanas like sirsha, sarvanga, etc., need not deter us from a choice of the asana for meditation.The Hatha Yoga prescribes several postures for different purposes.The book "Theory and Practices of Yoga Asanas "includes three parts namely Theory of Yoga Asanas in first part, Second part include Yoga Asanas Practice and last part included Yoga Asanas Postures.