Click in the first Heading 1 paragraph of your document. Click Home tab | Multilevel List | Define New Multilevel List (note that "define new" is misleading, because the same command should be used when you edit a multilevel list). In the multilevel list dialog box, type in Chapter followed by a space before the level 1 number. When you are done, click OK to exit the dialog box. The changes you make only apply to the active document.

For more on multilevel lists in general, take a look at Shauna Kelly's article at http://shaunakelly.com/word/numberin...g20072010.html.

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional