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Creating Files from Scratch
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ExcelApplication.Create uses an underlying template with defaults associated with the file format. XLS files use the defaults from Excel 2003; XLSX files use the defaults from Excel 2007. Switching to XLSX means the default template will change and the new defaults may change how output files look.
The largest impact from this relates to named styles. In Excel 2003, the default Normal style is Arial, size 10. In Excel 2007, the default Normal style is Calibri, size 11. Certain settings based on the Normal style, such as default row heights or column widths, will be different between XLS and XLSX files.
To ensure that output remains consistent while switching to XLSX, we recommend creating a blank XLSX file that has the Normal style set to Arial, size 10. Open that blank template file, rather than creating a template from scratch.