 Pyridazine

Pyridazine is the $1,2$-diazine: an aromatic six-membered heterocyclic compound with adjacent ring nitrogen atoms and formula ${\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{N}}_2$. The systematic ring position labels refer to connectivity, not to two added substituents on an unchanged hydrocarbon ring.^[ref]

Tracing once around the ring gives the atom sequence $\mathrm{N}-\mathrm{N}-\mathrm{C}-\mathrm{C}-\mathrm{C}-\mathrm{C}$. 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.^[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.