Chair
Chair is a bent-knee standing shape — like the bottom of a half-squat with the arms overhead. It loads the legs and stretches the underarms at the same time. Inside Sun Salutation B it's a single-breath shape; on its own it can be held for a few breaths.
How to do it
Stand tall with feet together or hip-width apart. Bend the knees as if sitting back into a low chair, sending the hips back behind you. Weight stays in the heels — if you can lift your toes, you've got it right. Sweep the arms overhead, palms shoulder-width apart, biceps near the ears. Tailbone lengthens down toward the floor; ribs stack over the hips.
Common mistakes
Don't let the knees track past the toes — sit back, not down. And don't compensate for tight shoulders by arching the lower back to get the arms overhead; let the arms stay slightly forward of vertical if that keeps the spine long.