Thomas MacKellar: The American Printer: A Manual of Typography: containing complete instructions for beginners, as well as practical directions for managing all departments of a printing office. MacKellar, Smiths & Jordan, Philadelphia 1870, p. 55 (Google BooksArchived 2023-03-26 at the Wayback Machine).
H. P. Trueman: The Eclectic Hand-book of Printing: Containing Practical Instructions to Learners; With Copious Quotations from Standard Works; Forming a Complete Guide to the Art of Printing. Second edition, Abel Heywood & Son, London 1880, p. 27 Google BooksArchived 2023-03-26 at the Wayback Machine),
Walter Thomas Rogers: A Manual of Bibliography: Being an Introduction to the Knowledge of Books, Library Management and the Art of Cataloguing, with a List of Bibliographical Works of Reference, a Latin-English and English-Latin Topographical Index of Ancient Printing Centres, and a Glossary. H. Grevel & Co., London 1891, p. 184 (Google BooksArchived 2023-03-26 at the Wayback Machine).
Thomas MacKellar: The American Printer: A Manual of Typography: containing complete instructions for beginners, as well as practical directions for managing all departments of a printing office. MacKellar, Smiths & Jordan, Philadelphia 1870, p. 55 (Google BooksArchived 2023-03-26 at the Wayback Machine).
H. P. Trueman: The Eclectic Hand-book of Printing: Containing Practical Instructions to Learners; With Copious Quotations from Standard Works; Forming a Complete Guide to the Art of Printing. Second edition, Abel Heywood & Son, London 1880, p. 27 Google BooksArchived 2023-03-26 at the Wayback Machine),
Walter Thomas Rogers: A Manual of Bibliography: Being an Introduction to the Knowledge of Books, Library Management and the Art of Cataloguing, with a List of Bibliographical Works of Reference, a Latin-English and English-Latin Topographical Index of Ancient Printing Centres, and a Glossary. H. Grevel & Co., London 1891, p. 184 (Google BooksArchived 2023-03-26 at the Wayback Machine).
United Nations Editorial Manual Online, IX. Footnote indicators, archived from the original on 2016-10-05, retrieved 2016-10-03