IMPORTANT! Sigil is continually updating, so if you are using the most recent version of SIGIL, then the SGF tool is located under “Edit” and is called “Insert Chapter Marker”.
Excerpt from my Ebook Formatting Kit 3.0; Splitting pages using Sigil SGF Chapter Insert and Chapter Break feature using Sigil 4.6 (Mac). Visit http://www.FirstStepsPublishing.com for more products guides, tips and videos.
NOTE: Sigil 4.9+ the SGF Chapter break is now under Edit in the menu bar.
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Thanks for your Tip.