D3 3.x was a monolithic library: the core functionality resided in a single repository and was published in a single file, d3.v3.js. It was possible to create a custom build using a [nonstandard tool](https://github.com/mbostock/smash), but not easy and few did. (There were also plugins, but these could only add features and had their own [monolithic repository](https://github.com/d3/d3-plugins).)
D3 4.0 is modular. Instead of one library, D3 is now [many small libraries](#table-of-contents) that are designed to work together. You can pick and choose which parts to use as you see fit. Each library is maintained in its own repository, allowing decentralized ownership and independent release cycles. Want to own a new repository in the [D3 organization](https://github.com/d3)? [Let me know!](https://twitter.com/mbostock)
D3 4.0 is modular. Instead of one library, D3 is now [many small libraries](#table-of-contents) that are designed to work together. You can pick and choose which parts to use as you see fit. Each library is maintained in its own repository, allowing decentralized ownership and independent release cycles.
The default bundle of D3 4.0 conveniently aggregates about thirty of these microlibraries.
