1At Sāvatthī. There the Blessed One said this:

"Bhikkhus, one thing, when developed and cultivated, is of great fruit and benefit. What one thing? Mindfulness of breathing. And how, bhikkhus, is mindfulness of breathing developed and cultivated so that it is of great fruit and benefit?

"Here, bhikkhus, a bhikkhu, having gone to the forest, to the foot of a tree, or to an empty hut, sits down. Having folded his legs crosswise, straightened his body, and set up mindfulness in front of him, just mindful he breathes in, mindful he breathes out. [n.289] What follows are the sixteen steps or aspects in the practice of mindfulness of breathing, which form the core of the Anapanasati Sutta (MN118). The sixteen steps are explained in detail at Vism 267–91 (Ppn 8:146–237), to which Spk refers the reader. A collection of important texts on this meditation subject, translated by Ñāṇamoli and entitled Mindfulness of Breathing, includes the Anapanasati Sutta, the passage from Vism, a treatise from Paṭis, and selected suttas.
As will be shown at SN54.10, the sixteen aspects fall into four tetrads, which are correlated with the four establishments of mindfulness. Thus, while mindfulness of breathing begins in the domain of "contemplation of the body" (kāyānupassanā), it eventually comprehends all four contemplations.
On the phrase "having set up mindfulness in front of him" (parimukhaṁ satiṁ upaṭṭhapetvā), Vibh 252,14–16 says: "This mindfulness is set up, well set up at the tip of the nose or at the centre of the upper lip."

2"Breathing in long, he knows: ‘I breathe in long’; or breathing out long, he knows: ‘I breathe out long.’ Breathing in short, he knows: ‘I breathe in short’; or breathing out short, he knows: ‘I breathe out short.’ He trains thus: ‘Experiencing the whole body, I will breathe in’; he trains thus: ‘Experiencing the whole body, I will breathe out.’ He trains thus: ‘Tranquillizing the bodily formation, I will breathe in’; he trains thus: ‘Tranquillizing the bodily formation, I will breathe out.’ [n.290] Vism 273–74 (Ppn 8:171–73) explains the third step of this tetrad to mean "making known, making plain, the beginning, middle, and end of the entire in-breath body … of the entire out-breath body." The "bodily formation" (kāyasaṅkhāra), in the fourth step, is the in-and-out breathing itself, which becomes progressively calmer and more subtle as mindfulness of the breath develops. See SN41.6: Assāsapassāsā kho gahapati kāyasaṅkhāro, "In-breathing and out-breathing, householder, are the bodily formation." sn.v.312

3"He trains thus: ‘Experiencing rapture, I will breathe in’; he trains thus: ‘Experiencing rapture, I will breathe out.’ He trains thus: ‘Experiencing happiness, I will breathe in’; he trains thus: ‘Experiencing happiness, I will breathe out.’ He trains thus: ‘Experiencing the mental formation, I will breathe in’; he trains thus: ‘Experiencing the mental formation, I will breathe out.’ He trains thus: ‘Tranquillizing the mental formation, I will breathe in’; he trains thus: ‘Tranquillizing the mental formation, I will breathe out.’ [n.291] This note and the two to follow are based on Vism 287–91 (Ppn 8:226–37).
Rapture (pīti) is experienced when he has entered upon the lower two jhānas and when, after entering upon and emerging from one of those jhānas, he comprehends with insight the rapture associated with the jhāna as subject to destruction and vanishing. Happiness (sukha) is experienced when he has entered upon the lower three jhānas and when, after entering upon and emerging from one of those jhānas, he comprehends with insight the happiness associated with the jhāna as subject to destruction and vanishing. The mental formation (cittasaṅkhāra) is feeling and perception, which are experienced in all four jhānas.

4"He trains thus: ‘Experiencing the mind, I will breathe in’; he trains thus: ‘Experiencing the mind, I will breathe out.’ He trains thus: ‘Gladdening the mind, I will breathe in’; he trains thus: ‘Gladdening the mind, I will breathe out.’ He trains thus: ‘Concentrating the mind, I will breathe in’; he trains thus: ‘Concentrating the mind, I will breathe out.’ He trains thus: ‘Liberating the mind, I will breathe in’; he trains thus: ‘Liberating the mind, I will breathe out.’ [n.292] "Experiencing the mind" is to be understood by way of the four jhānas. The mind is "gladdened" by the attainment of the two jhānas accompanied by rapture or by the penetration of these with insight as subject to destruction and vanishing. "Concentrating the mind" refers either to the concentration of the jhāna or to the momentary concentration that arises along with insight. "Liberating the mind" means liberating it from the hindrances and grosser jhāna factors by attaining successively higher levels of concentration, and from the distortions of cognition by way of insight knowledge.

5"He trains thus: ‘Contemplating impermanence, I will breathe in’; he trains thus: ‘Contemplating impermanence, I will breathe out.’ He trains thus: ‘Contemplating fading away, I will breathe in’; he trains thus: ‘Contemplating fading away, I will breathe out.’ He trains thus: ‘Contemplating cessation, I will breathe in’; he trains thus: ‘Contemplating cessation, I will breathe out.’ He trains thus: ‘Contemplating relinquishment, I will breathe in’; he trains thus: ‘Contemplating relinquishment, I will breathe out.’ [n.293] "Contemplating impermanence" (aniccānupassi) is contemplation of the five aggregates as impermanent because they undergo rise and fall and change, or because they undergo momentary dissolution. This tetrad deals entirely with insight, unlike the other three, which can be interpreted by way of both serenity and insight. "Contemplating fading away" (virāgānupassī) and "contemplating cessation" (nirodhānupassi) can be understood both as the insight into the momentary destruction and cessation of phenomena and as the supramundane path, which realizes Nibbāna as the fading away of lust (virāga, dispassion) and the cessation of formations. "Contemplating relinquishment" (paṭinissaggānupassi) is the giving up (pariccāga) or abandoning (pahāna) of defilements through insight and the entering into (pakkhandana) Nibbāna by attainment of the path. See n. 7.

6"It is, bhikkhus, when mindfulness of breathing is developed and cultivated in this way that it is of great fruit and benefit."

1Sāvatthinidānaṁ. Tatra kho … pe … etadavoca:

"Ekadhammo, bhikkhave, bhāvito bahulīkato mahapphalo hoti mahānisaṁso. Katamo ekadhammo? Ānāpānassati. Kathaṁ bhāvitā ca, bhikkhave, ānāpānassati kathaṁ bahulīkatā mahapphalā hoti mahānisaṁsā?

Idha, bhikkhave, bhikkhu araññagato vā rukkhamūlagato vā suññāgāragato vā nisīdati pallaṅkaṁ ābhujitvā ujuṁ kāyaṁ paṇidhāya parimukhaṁ satiṁ upaṭṭhapetvā. So satova assasati, satova passasati.

2Dīghaṁ vā assasanto ‘dīghaṁ assasāmī’ti pajānāti, dīghaṁ vā passasanto ‘dīghaṁ passasāmī’ti pajānāti; rassaṁ vā assasanto ‘rassaṁ assasāmī’ti pajānāti, rassaṁ vā passasanto ‘rassaṁ passasāmī’ti pajānāti; ‘sabbakāyappaṭisaṁvedī assasissāmī’ti sikkhati, ‘sabbakāyappaṭisaṁvedī passasissāmī’ti sikkhati; ‘passambhayaṁ kāyasaṅkhāraṁ assasissāmī’ti sikkhati, ‘passambhayaṁ kāyasaṅkhāraṁ passasissāmī’ti sikkhati.

3‘Pītippaṭisaṁvedī assasissāmī’ti sikkhati, ‘pītippaṭisaṁvedī passasissāmī’ti sikkhati; ‘sukhappaṭisaṁvedī assasissāmī’ti sikkhati, ‘sukhappaṭisaṁvedī passasissāmī’ti sikkhati; ‘cittasaṅkhārappaṭisaṁvedī assasissāmī’ti sikkhati, ‘cittasaṅkhārappaṭisaṁvedī passasissāmī’ti sikkhati; ‘passambhayaṁ cittasaṅkhāraṁ assasissāmī’ti sikkhati, ‘passambhayaṁ cittasaṅkhāraṁ passasissāmī’ti sikkhati.

4‘Cittappaṭisaṁvedī assasissāmī’ti sikkhati, ‘cittappaṭisaṁvedī passasissāmī’ti sikkhati; ‘abhippamodayaṁ cittaṁ assasissāmī’ti sikkhati, ‘abhippamodayaṁ cittaṁ passasissāmī’ti sikkhati; ‘samādahaṁ cittaṁ assasissāmī’ti sikkhati, ‘samādahaṁ cittaṁ passasissāmī’ti sikkhati; ‘vimocayaṁ cittaṁ assasissāmī’ti sikkhati, ‘vimocayaṁ cittaṁ passasissāmī’ti sikkhati; ‘aniccānupassī assasissāmī’ti sikkhati.

5‘Aniccānupassī passasissāmī’ti sikkhati; ‘virāgānupassī assasissāmī’ti sikkhati, ‘virāgānupassī passasissāmī’ti sikkhati; ‘nirodhānupassī assasissāmī’ti sikkhati, ‘nirodhānupassī passasissāmī’ti sikkhati; ‘paṭinissaggānupassī assasissāmī’ti sikkhati, ‘paṭinissaggānupassī passasissāmī’ti sikkhati.

6Evaṁ bhāvitā kho, bhikkhave, ānāpānassati evaṁ bahulīkatā mahapphalā hoti mahānisaṁsā"ti.

Paṭhamaṁ.