The first school that genuinely grew on Chinese soil.
Before Tiantai, Chinese Buddhism was largely occupied with digesting what had arrived from India. Zhiyi did something nobody before him had done: he put the entire body of scripture translated up to that point into a new order and gave it one unified framework of interpretation.
The framework is called "the five periods and eight teachings". It solves a very practical problem. The sutras contradict one another, so which do you believe? Tiantai answers: all of them, spoken to people at different stages.
Three thousand realms in a single thought: one moment of mind already holds the three thousand realms complete (ten realms mutually containing × ten suchnesses × three worlds). The claim is mutual inclusion. No single thought stands alone, each one holds the whole. It is a statement about how things contain each other, and it is not an idealism in which mind manufactures the world.
The three truths in perfect interfusion: emptiness, the provisional and the middle are three faces of one present moment rather than three levels stacked up. The threefold contemplation in a single mind means seeing all three faces within one thought.
The five periods and eight teachings: the Buddha's teaching is sorted into five periods (Avataṃsaka, Āgama, Vaipulya, Prajñā, and Lotus with Nirvāṇa), and the teachings into four modes of conversion and four doctrinal categories. The Lotus Sutra is ranked as the perfect teaching, and the school is built on that ranking.
Ranked by Tiantai as the highest, the perfect teaching. "Merging the three vehicles into one" and "opening the provisional to reveal the real" are where the whole classification lands.
Nominally an explanation of the sutra's title. In practice a complete system of Buddhist philosophy.
A line-by-line commentary on the Lotus Sutra.
The summit of Tiantai practice theory and the full account of perfect and sudden stilling and insight. The most systematic work on meditation in East Asian Buddhism, with nothing else close.
The old practice manual this site recommends above all others. It runs from regulating food and sleep through regulating the body and the breath to regulating the mind, right down to posture and breathing. "Fixing attention at the nostrils" comes from here.
Counting, following, stilling, observing, turning back, purity. The breath methods in section 04 of the front page come out of this text. Worth noting: the six items were already in place in the Han-dynasty ānāpāna literature. Zhiyi systematised them.
The core of the Mohe Zhiguan: no graded sequence, direct contemplation of the three interfused truths within the present thought.
Counting, following, stilling, observing, turning back, purity. The most usable and most widely transmitted set of breath methods there is, and one of the East Asian sources of the mindfulness-of-breathing practice done worldwide today.
A Tiantai classification that brings every posture of practice into one scheme. The constantly walking samādhi, ninety days of circumambulation reciting the Buddha's name without stopping, is known for sheer intensity.
Identity in principle, in name, in contemplative practice, in resemblance, in partial realisation, in ultimacy. Tiantai's instrument against overrating yourself. You use it to work out which rung you are actually on, so that an early experience does not get filed as awakening. A very clear-eyed piece of work, and worth looking at on its own.
"Three thousand realms in a single thought" is a claim of mutual inclusion: every thought holds the whole and no dharma stands alone. It belongs to ontology and to interpretation. It does not say that thoughts create the world.
Tiantai has the most solid body of practical literature in East Asian Buddhism. The operational detail in the Mohe Zhiguan and the Lesser Zhiguan is a great deal more concrete than anything in the Chan (Zen) records.
If you want to take sitting seriously, start with the Lesser Zhiguan. It is the old practice manual this site recommends most.
If you want to understand why the sutras say different things, read an account of the five periods and eight teachings. Even if you reject Tiantai's conclusions, its method of classification is a genuinely useful way of handling contradictory texts.