The teaching of chemistry inherently relies on models to represent the underlying molecular processes, structures, properties, interactions, behaviors, and physics which drive phenomenological chemical change. In a chemistry context, the importance of models derives from the fact that atomic level molecular changes are usually not possible to observe directly. In constructing a model, the choice of representation depends on the size and complexity of the molecular structure being examined; as complexity grows, it is increasingly important to have compact models for abstracting the structure to make it intelligible. Early in their chemical education, students are taught to use Lewis structures as molecular representations, which are shortly thereafter replaced by skeletal structures as the molecules become more complex. In typical first-year undergraduate classes, students are introduced to new representations, such as Newman projections, which are designed to capture 3D information in 2D. In the teaching of molecular symmetry, it is common to rely on physical 3D molecular model construction kits, which can be used to intuitively illustrate symmetry operations in a fashion that engages 3D spatial reasoning. In structural biology, coarse representations called ribbon or cartoon diagrams (also called Richardson diagrams) are used to help simplify the visualisation of protein secondary structure.