Present participle of diagram; the act of creating or representing something using a diagram (British spelling variant with doubled m).
From diagram plus -mming (doubled m plus -ing), following British conventions. Standard present participle in British English.
Sentence diagramming was once taught in English classes as a way to understand grammar visually—students would draw lines showing which words modified which others. It fell out of fashion, but the technique still works: making grammar visible helps brains understand abstract relationships.
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