SASS how to add styling to all classes that preceed with X

I saw this on a Treehouse lesson earlier and wanted to keep it in mind for my future CSS code. What it does is add my chosen styling to all the classes that preceed with the chosen markup.

SASS Placeholder selector

I've found this nugget of information hard to find on Google for some reason.

All I need to is make a placeholder and call it up again.

/* Placeholder selector */

%my-placeholder {

  border: 1px solid black;

}




p {

@extend %my-placeholder;

}