A source that is entirely dependent on secondary sources or primary and secondary sources for its existence is a tertiary source. Sources like ‘guides to reference sources’ and ‘bibliography of bibliographies’ are examples of tertiary sources. These sources act as key to primary sources as well as secondary sources.
Some authors have considered directories, yearbooks, etc. also as tertiary sources as they help the searcher in using primary and secondary sources. There are sources, like directory of on-going research projects, which are placed under tertiary sources.