.xls File — Microsoft Excel Spreadsheet (Legacy)

XLS is the binary file format used by Microsoft Excel from versions 97 through 2003, storing spreadsheet data in the Binary Interchange File Format (BIFF). XLS files contain worksheets organized into rows and columns, with support for formulas, charts, pivot tables, conditional formatting, macros, and external data connections. The format uses the OLE2 compound document structure, which encapsulates multiple data streams within a single file. XLS files can contain up to 65,536 rows and 256 columns per worksheet, a limitation that was expanded in the later XLSX format. The format supports VBA macros for automation, making it powerful for business applications but also susceptible to macro-based malware. Despite being superseded by XLSX, many legacy business systems and financial applications still produce and consume XLS files. Data integrity and formula compatibility remain strong across implementations.