Quaternion is described as 'cross-platform Qt5-based desktop IM client for Matrix, a reference desktop application using libQMatrixClient. Source code, issue tracker, roadmap etc. can be found at GitHub' and is an app in the Social & Communications category. There are more than 25 alternatives to Quaternion for a variety of platforms, including Android, iPhone, Mac, Windows and Linux. The best alternative is Telegram, which is free. Other great apps like Quaternion are Signal (Free, Open Source), Element (Freemium, Open Source), Tox (Free, Open Source) and Wire (Free Personal, Open Source).
Signal is a messaging app with an unexpected focus on privacy, combined with all the features expected from a modern IM app. Signal has received a lot of praise from all around the world for its security and privacy-respecting technologies.
Element is a secure collaboration and messaging app based on the Matrix protocol. It’s decentralised to deliver digital sovereignty, and based on an open standard for easy connections and interoperability. It is end-to-end encrypted by default.
Wire is the most secure collaboration platform featuring messenger, voice, video, conference calls, file-sharing, and external collaboration –all protected by the most secure end-to-end-encryption. Why Wire?
WhatsApp is an iPhone and Android messaging app which allows you to exchange messages without having to pay for SMS. In addition to basic messaging WhatsApp users can create groups, send each other unlimited images, video and audio media messages.
An open-source decentralized and encrypted messaging app even without internet connection via Wi-Fi and Bluetooth. Messages are stored securely on your device, not in the cloud. Connect directly with nearby contacts - no Internet access is required.