The Pycao Software. Handling three-dimensional objects is required for mechanical design, thermal study, mathematical illustrations... Transferring objects between applications is possible using plug-ins and a common specification. Many low level specifications, like wavefront .obj, use coordinates. In contrast, Pycao is a language developed as a python module to avoid coordinates as much as possible. The primary focus is on geometrical concepts for the most concise possible description of the objects. The paradigms include the classical framework of affine geometry, carrying objects inside boxes, various frames attached to those boxes, markers glued to the objects, and a class to build libraries. In the backend, the computations take place in a massive space, to globalize the affine and linear constructions, and to handle barycenters easily. A ”natural geometrical language” for 3d-objects is still to be defined by consensus as an aggregation of concepts from various pieces of software. Pycao is a step in this direction. The goal of the talk is to describe the concepts used in the pycao software, both from the end-user and from the developer point of view. The pycao language, with documentation and a povray plug-in can be downloaded: evain/software/softwareIntro.html