What’s the difference? Simply put, in a thin registry, the contact details for a domain registration (namely the registrant, admin, technical and billing contacts) are stored at the registrar, and the registry’s whoisserver only shows basic domain information, and provides a referral to the “registrar whois” which then shows the relevant contact data. Conversely, in a thick registry system, the contact details are stored at the registry level, and are shown in the “registry whois”. There is no “registrar whois”.