Diagonal is a steward-owned data science consultancy working with projects in the built environment. We build interactive tools to help people understand the tradeoffs inherent in their plans to evolve cities. Our tools are powered by B6 - an in-memory geospatial analysis engine we built to work with large data sets describing the built environment. We typically use it work work with OpenStreetMap and open government data. To enable others to repeat our analyses, we recently released B6 as open source. In this talk, we'll give an overview of B6, including how it's implemented, and how we use it in our commercial work.