Too bad Twitter needs their own propriety tags and doesn't just use ogg tags for images like FB and Google+.I was thinking the same thing when I saw the proprietary twitter card markup. We're talking about Open Graph tags (abbrev OG or og). Apparently there's an overlap in some core og markup for title, description, image, etc... but Twitter, Facebook, Google+, Medium, Pinterest, LinkedIn, etc are all adding proprietary tags. It would be great to have one simplified standard... though perhaps that's no longer possible.
The table in this section explains the OpenGraph fallback behavior for each Twitter tag.