The Sandwich Parisien Classique is the codified version of the everyday Paris baguette sandwich: the same idea as the generic Sandwich Parisien, but settled into a fixed composition rather than left loose to the counter's whim. The build is specific. A split baguette, a thick layer of barely-salted butter on both faces of the crumb, jambon de Paris cut to order and laid in folded shingles, and the canonical optional additions kept in their proper place: a leaf of crisp lettuce, a thin slice of ripe tomato, sometimes cornichons, but on the side rather than inside. It is the orthodox build, one step up from the stripped Jambon-Beurre, where the additions are not improvised but understood, each with a fixed role.
The craft is in why the codification holds. Every element in the classic build answers another, which is the reason the composition stabilized into a recognized form rather than staying arbitrary. The butter rounds the ham's gentle cure and seals the crumb against the tomato's water; the lettuce supplies a cold, clean crunch against the soft ham; the cornichon's vinegar lives on the side precisely because inside it would cut across the butter-and-ham contrast that the whole sandwich is built on. The bread does the structural work and has to be that morning's baguette, crackling outside and open inside. The discipline is proportion and placement: the classic is not the loose version with more in it, it is the loose version with everything in its right amount and the acid kept out of the core.
Variations are the regional accents on a fixed frame rather than reinventions of it. Jambon de Bayonne, the dry-cured southwestern ham, sometimes stands in for jambon de Paris; a thin layer of Comté or young Cantal is added in cheese country; pain de seigle replaces the baguette in northern bakeries. Each is recognizably the same codified sandwich spoken with a regional inflection, the composition held constant and only the components swapped. It belongs with the place-keyed sandwiches the catalog groups under Regional Specialty Sandwiches, and its specific contribution is to be the orthodox, settled build of the Paris baguette sandwich, the fixed reference the everyday version drifts around.