Cornerstone17 was established as a practical reference tool for navigating the contemporary web. Where search engines surface results by algorithmic rank, Cornerstone17 takes a different approach: curation, organisation, and consistent subject filing. Each of the 829 entries in this catalogue has been reviewed before inclusion and assigned to one of twenty-two subject registries, from the Finance & Markets Ledger to the Travel & Destinations Catalogue. The index draws its name from a simple principle: that well-organised information is the cornerstone of useful research. In an era of abundant content and fractured attention, a curated catalogue offers something search cannot — a coherent, browsable structure maintained by an editorial hand rather than a ranking algorithm. Cornerstone17 accepts submissions from site owners seeking inclusion. Submitted sites are reviewed against the catalogue's subject scope and, if accepted, filed under the appropriate registry. There is no fee for listing. The review process is conducted by the catalogue's curators, and decisions are final. The catalogue covers a broad range of subject areas, including health and clinical services, legal and advocacy practices, technology and software providers, and the open acquisitions section, which gathers sites that do not fit neatly within any single dedicated registry. This breadth is intentional: a complete reference index should reflect the full diversity of the web, not only its most familiar corners. Cornerstone17 is not affiliated with any of its listed entries and does not accept payment for placement or ranking. It operates as an independent reference catalogue, maintained in the interest of an organised and navigable web.