Essential requirements in tai chi that must be fully understood and practiced in order to progress in this art.
The thirteen requirements (as spelled out in tai chi classics by Wu Yuxiang, etc.) consist of the eight torso methods and the five essential requirements:
- Eight Torso Methods:
- Holding in the chest
- stretching the back
- keeping the head upright (suspending the head top)
- suspending the crotch
- loosening the shoulders
- dropping of elbows
- wrapping the crotch
- protecting the stomach (or upper abdomen)
- Five Essential Requirements:
- Keeping the body upright
- distinguishing between substantiality and insubstantiality
- sinking qi down to dantian
- attentive spirits
- martial spirits

Read Quintessence for further detail here.