Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Pair is a mobile app which provides a mobile messaging service for two people, especially romantic couples. Like many mobile phone messaging applications, Pair allows users to share text, photos, video and other content. It's a competitor to apps like WhatsApp, Facebook Messenger, and KakaoTalk, but it's unique in that Pair is for communicating with exactly one other person. Pair is one of a slew of mobile applications that intentionally confine their communication to a small group, as opposed to large public or semi-public networks like Twitter. The app is currently available on iPhone and Android. The free application garnered more than 100,000 users after only a week. TenthBit, the company producing Pair, is part of the Y Combinator startup incubator.