We use horizontal lines before and after advertisements to signify breaks in our text, so they don’t appear like images we’ve added to our posts.
We found some HTML that did the job.
<hr class="wp-block-separator"/>
However, we wanted the wide line and ran a quick Google to find the modifier. Didn’t find it as fast as we’d like. It occurred to us that we could simply
switch between WordPress’ visual editor and code editor
to see what the modifier was. Bingo.
<hr class="wp-block-separator is-style-wide"/>