Copy editors require additional information in order to perfect the editing process. A stylesheet is a tool used by editors and proofreaders to ensure consistency of language, accents, grammar preferences, hyphenated words, jargon, spelling of names, acronyms and even abbreviations.
For example, English copy can be edited for American or British English and the way speech is presented for each is different. For book editing, stylesheets can also include character, place and plot details.
An editor and proofreader will develop a stylesheet for each project and agree to the language and formatting principles with their clients upfront. This helps guide the editor in making changes that are consistent and sensical. A stylesheet can also include information pertinent to the copy and its audience particularly that will help contextualise the text. The more complex and technical the copy the more extensive and detailed the stylesheet will be. Clients should sign off on the stylesheet together with the editor or proofreader as a failsafe in creating excellence in the work.
A stylesheet can also good to create common understanding of what will be edited and proofread for correction. For example, proofreaders cannot check spelling of names without client confirmation as there are so many different ways they are written, the same applies for names of companies, places and 21st century terminology.
Having an agreed stylesheet from inception helps streamline the process for both client and editor/proofreader and creates a form of assurance for both.