A paragraph indent is basically a space or several spaces reserved from the margin before the starting line of text in a paragraph.
The lines of text in a paragraph are indented depending on the type of document being made. Rules may vary depending on the type, but generally, the first line of a paragraph is indented to mark its beginning. This is helpful as it can be hard to spot the start of a paragraph when all the lines of text start at the same level. There are also other kinds of indentations like the "hanging indent" for the lines of text below the first.
Paragraph indentation is basically a mark of a good layout for a document. Those who use it give a professional appearance to the work they create.
Using paragraph indentations
The following screencasts demonstrate how to use paragraph indentations:
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