Why can’t I find the margin tool?
Please be more descriptive: are talking about page margins or paragraph margins also called indents?
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I would make a guess that you mean a device called margin tool in MS Word. I have forgotten most of what I knew about Word (and have felt much better ever since). but I think it did much the same job as the triangles on what LO calls the horizontal ruler. I do not think I can explain these better than LO’s help (at Using Rulers - LibreOffice Help)