There are sixteen instructions of mindfulness in the Ānāpānasati Sutta that are further broken down into four groups or Tetrads as follows (taken from Wikipedia):

1. First Tetrad: Contemplation of the Body (kāya)
   1. Discerning the in and out breathing
   2. Discerning long or short breaths
   3. Experiencing the whole body (sabbakāya).
   4. Calming bodily formations (kāya-saṃskāra)
2. Second Tetrad: Contemplation of the Feeling (vedanā)
   1. Experiencing rapture (pīti)^[15]
   2. Experiencing pleasure (sukha)
   3. Experiencing mental fabrication (citta-saṃskāra)
   4. Calming mental fabrication
3. Third Tetrad: Contemplation of the Mind (citta)
   1. Experiencing the mind
   2. Satisfying the mind
   3. Steadying the mind (samādhi)
   4. Releasing the mind
4. Fourth Tetrad: Contemplation of the Mental Objects (dhammā)
   1. Dwelling on impermanence
   2. Dwelling on dispassion (virāga).
   3. Dwelling on cessation (nirodha).
   4. Dwelling on relinquishment (paṭinissaggā).