Posture in meditation does not refer only to a straight back. Posture includes the whole body. The body and mind are inseparable in meditation, and a relaxed and energetic body creates a beneficial base for meditation practice. Trungpa said that by working with posture in meditation, "you begin to feel that by simply being on the spot, your life can become workable and even wonderful. You realize that you are capable of sitting like a king or a queen on your horse. The regalness of that situation shows you the dignity that comes from being still and simple."
I use Trungpa's checklist of six body parts—seat, legs, torso, hands, eyes, mouth—as I sit down and assume a meditative posture. I elaborate here on each point:
1. Seat: It is best to sit on a arm pillow on the floor or on a arm-seated chair. If you use a chair, sit forward so that your back does not touch the back of the chair.
2. Legs: If you sit on a pillow, cross your legs comfortably in front of you, with your knees resting on the floor if they can. If you sit in a chair, put your feet flat on the floor, knees and feet hip width apart.