Medium is a public blogging platform designed by Twitter founder Evan Williams and Obvious to encourage reader-writer collaboration. Medium users can write, comment, and contribute to posts, organize collections of content, or simply read what others have posted. The site is designed to be complementary to Twitter, and only those with a Twitter account can join.

The beta version of Medium debuted in August 2012, and its most striking feature is its simplicity. It purposely elides plug-ins and sidebars, and as Williams’ introductory note states, “there is nothing to set up or customize.” With ample white space, a curated home page of the best — not the most recent — content, and a built-in pre-publish collaboration tool, the goal of Medium is to reinvent how prose is composed and experienced online. Observers have questioned the purpose of Medium, debating whether it’s intended to be similar to a curated magazine, a more open blogging platform or something in between. Williams has described Medium as both a platform for individuals to publish and a publisher itself.

The Medium team is composed of alumni from past Obvious ventures, including Twitter, Odeo, and Blogger. It has mostly been self-funded, but raised its first outside capital – $25 million – in 2014. On occasion, its editorial team commissions posts and pays authors for contributing, and they also accept pitches from experienced journalists for investigative pieces. It has been criticized by some writers for its pay-per-click policy. In 2013, Medium opened access to everyone.

Medium bought the science journalism startup Matter in 2013 and relaunched it in 2014 as an online magazine. It hired tech writer Steven Levy from Wired in 2014. It also planned to launch a music magazine later that year.

Obvious has also partnered with Branch, a content-sharing venture designed by Josh Miller, Cemre Güngöre, and Hursh Agrawal that came out of beta in August 2012. Originally called Roundtable, Branch aims to incubate “high quality public discourse” by allowing users to “host” discussions that they can invite their friends to join. Branch is also behind Potluck, a link-sharing platform. Both were bought by Facebook in 2014.