In modern spreadsheet applications, several spreadsheets, often known as "worksheets" or simply "sheets", are gathered together to form a "workbook". A workbook is physically represented by a file, containing all the data for the book, the sheets, and the cells with the sheets. Worksheets are normally represented by tabs that flip between pages, each one containing one of the sheets, although Numbers changes this model significantly. Cells in a multi-sheet book add the sheet name to their reference, for instance, "Sheet 1!C10". Some systems extend this syntax to allow cell references to different workbooks.