Not every French sandwich is on a baguette. The Pain Garni category covers the long shelf of other French breads — pain de campagne (the round country loaf), pain au levain (sourdough), pain au seigle (rye, often Alsatian), pain de mie (the soft white sandwich loaf), pain aux céréales (multigrain), pain aux noix (walnut), pain aux raisins, pain au sésame, pain Poilâne (the famously dense Parisian wheel) — and the sandwiches made on them are the ones the boulangerie sells in addition to the jambon-beurre.
The choice of bread is mostly a textural one. Pain de campagne is dense, slightly sour, has a thick crust, and works for sandwiches where the filling is robust enough to push back — long-aged cheese, country pâté, smoked salmon with butter. Pain au levain trades a little crust for more open crumb and is the right base for a Saumon-Avocat or a Salade de Lentilles. Pain de mie is the softest of all, has almost no crust, and is the bread of the tea-room sandwich — cucumber, watercress, butter, crusts trimmed.
The seeded and grain breads — Céréales, Noix, Sésame, Seigle — push the sandwich further toward the Northern European tradition. They pair best with smoked fish, with fresh white cheeses, with the kinds of vegetables that need an assertive bread underneath them. The pain de mie pushes the sandwich in the opposite direction, toward the British finger sandwich and the Japanese sando — and shows up most often at French afternoon tea, where the goal is delicacy rather than substance.