What is the difference between an inlay, an onlay, an overlay and a crown?

What the drawing is measured from

1. Preparation dimensions follow concordant published guidelines: pulpal floor 1.5 to 2.0 mm, isthmus 2.0 mm, a 90 degree butt-joint cavosurface with no bevel, rounded internal line angles, and walls flared 10 to 12 degrees in total. Cuspal reduction 1.5 to 2.0 mm, drawn here at the shallow end of that range.
2. Covering a cusp is advised below roughly 2 mm of remaining thickness at the base of a supporting cusp and 3 mm at a non-supporting one. In a lower molar the buccal cusps support and work in compression, while the lingual cusps work in tension and are the ones that fail, which is why the onlay here covers the lingual cusp.
3. Those coverage thresholds date from the era of metal restorations and are being revisited. Bonded lithium disilicate onlays left on thin cusps did not fracture under five clinical years of chewing simulation, and the authors argue for preparations that keep more tooth. Griffis et al. 2020, Journal of Esthetic and Restorative Dentistry. DOI
4. Bonded ceramic inlays, onlays and overlays survived 92 to 95 per cent at five years across 5,811 restorations, and 91 per cent at ten years across 2,154, with fracture the commonest failure and endodontic complications the next. Systematic review and meta-analysis, Morimoto et al. 2016, Journal of Dental Research. DOI
5. Whole-tooth volumes, for comparison with the section areas on the drawing: adhesive and inlay preparations removed 5.5 to 27.2 per cent of the coronal tooth and complete crowns 67.5 to 75.6 per cent. Measured gravimetrically on resin replica teeth, 10 preparations per design, not on natural teeth. Edelhoff and Sorensen 2002, PMID 12186346. PubMed
6. Enamel thickness at named occlusal landmarks, from cone-beam CT at 0.16 mm voxel across 45 mandibular first permanent molars: mesiobuccal cusp tip 1.58 mm, mesiolingual 1.40 mm, both marginal ridges 1.48 mm, and the central fossa 0.98 mm, the thinnest landmark measured. Zhang et al. 2026, BMC Oral Health. DOI
7. Which restoration suits a given tooth is a clinical judgement made case by case, on the tooth in front of you.