YouTube Play Button

YouTube Play Buttons, a part of the YouTube Creator Rewards, are a recognition by YouTube of its most popular channels. These are distinct from YouTube Awards which are intended to recognize the best quality videos; the YouTube Creator Rewards are based solely on a channel's subscriber count.

When a verified YouTuber reaches a specific milestone, they are awarded a relatively flat trophy with plastic (diamond, ruby) or metal (silver, gold) shaped like the YouTube play button symbol. The trophies are of different sizes: each button and plaque get progressively bigger the more subscribers the channel gets.

There are currently three levels of rewards:

YouTuber, PewDiePie, received a ruby trophy in the shape of his channel's logo as a gift for reaching 50,000,000 subscribers. It isn't referenced in the Creators Award page and it is unknown what will be given out to any other YouTuber who surpasses the milestone in the future.

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Wikipedia