Thumbs Up for Duet Display
macsparky.com/blog/2015/12/thumbs-up-for-duet-display
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
There is a wonderful little application called Duet, made by some former Apple engineers, that lets you connect your iPad to your Mac and turn the iPad into a second screen. I’m now doing this often.