Line starts with underscore
_This for example
<i>This</i> for example

Line starts with double underscore
__This, for example
<i>This,</i> for example

Line has embedded underscores
This has some_embedded_underscores
This has some_embedded_underscores

Doubled underscore, midline
This is __doubled, it seems
This is <i>doubled,</i> it seems

Line has escaped underscores
This has some\_escaped\_underscores
This has some_escaped_underscores