Relative pronouns indtroduce relative clauses.

• who
• whom
• whose
• which
• that
• where: '(places) in/at which'
• when: '(times) on which'
• why: '(reasons) for which'

https://learnenglish.britishcouncil.org/en/english-grammar/pronouns/relative-pronouns http://dictionary.cambridge.org/grammar/british-grammar/pronouns/relative-pronouns