= Pyridazine
<Pyridazine> is the $1,2$-<diazine>: an aromatic six-membered <heterocyclic compound> with adjacent ring <nitrogen> atoms and formula $\mathrm{C_4H_4N_2}$. The systematic ring position labels refer to connectivity, not to two added substituents on an unchanged hydrocarbon ring.<PubChem Compound database by National Center for Biotechnology Information>[Pyridazine CID9259; 2D structure, formula, 1,2-diazine synonym and connectivity descriptors]{ref}
Tracing once around the ring gives the atom sequence $\mathrm{N-N-C-C-C-C}$. In <pyrimidine>, the shortest path between the two <nitrogen> atoms includes one <carbon>; in <pyrazine>, it includes two. Those are different <constitutional isomers>, although all three have the same elemental formula.<PubChem Compound database by National Center for Biotechnology Information>[Compare CID9259, CID9260 and CID9261 structures]{ref}
For a structure-reading exercise, start with a hexagon marked with two adjacent $\mathrm{N}$ labels and rotate the entire page. It remains <pyridazine>. Move one $\mathrm{N}$ label across a ring vertex instead, while keeping the ring connectivity interpretation, and the identity can become <pyrimidine>. The useful distinction is between moving a picture and changing a <molecule>.
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