ALTO (Analyzed Layout and Text Object) is a XML Schema that details technical metadata for describing the layout and content of physical text resources, such as pages of a book or a newspaper. It most commonly serves as an extension schema used with METS. However, ALTO instances can also exist as a standalone document used independently of METS.
Each ALTO file contains a style section where different styles (for paragraphs and fonts) are listed. The layout section contains what’s on the page. A page is divided into several regions (Print space, left margin, right margin, top margin and bottom margin). For each region all objects are listed which have been detected inside.