Hart's Rules for Compositors and Readers at the University Press, Oxford—today published under the short title New Hart's Rules—is an authoritative reference book and style guide published in England by Oxford University Press (OUP). Hart's Rules originated as a compilation of best practices and standards by English printer and biographer Horace Hart over almost three decades during his employment at other printing establishments, but they were first printed as a single broadsheet page for in-house use by the OUP in 1893 while Hart's job was controller of the university press. They were originally intended as a concise style guide for the staff of the OUP, but they developed continuously over the years, were published in 1904, and soon gained wider use as a source for authoritative instructions on typesetting style, grammar, punctuation, and usage.
Hart's Rules has been revised and republished under different titles, including The Oxford Guide to Style (2002), The Oxford Style Manual (2003, also including The Oxford Dictionary for Writers and Editors of 2000), New Hart's Rules (2005, an updated but abridged, pocket-size version), and New Oxford Style Manual (2012, inclusive of New Hart's Rules and The New Oxford Dictionary for Writers and Editors of 2005, together notably shorter than the 2003 combined edition). A revised second edition of New Hart's Rules (without the Dictionary) was released in 2014, and a second New Oxford Style Manual was compiled in 2016, using the 2014 versions of both of the individual volumes.