morphisms in monoidal categories can also be drawn as string diagrams since a strict monoidal category can be seen as a 2-category with only one object (there will therefore be only one type of planar region) and mac lane's strictification theorem states that any monoidal category is monoidally equivalent to a strict one. the graphical language of string diagrams for monoidal categories may be extended to represent expressions in categories with other structure, such as braided monoidal categories, dagger categories, etc. and is related to geometric presentations for braided monoidal categories and ribbon categories. in quantum computing, there are several diagrammatic languages based on string diagrams for reasoning about linear maps between qubits, the most well-known of which is the zx-calculus.