The United States Department of Justice filed an antitrust lawsuitagainst Apple earlier today. The suit touches on several (allegedly) anticompetitive practices. But one category in particular caught our attention as the DOJ spends quite a bit of time talking about “green bubbles” and “blue bubbles.”

When an iPhone user sends a message to another iPhone user, by default that message is sent using Apple’s iMessage protocol. This protocol offers several advantages, such as end-to-end encryption (although iCloud backups are not encrypted by default, but that’s another story), read receipts, typing indicators and high-quality photos and videos.