Casse-Croûte is the French word for the sandwich you eat between meals when you can't be bothered with a proper one. The literal translation is "break the crust," and the term covers the snack-sandwich category the way "snack" covers an entire shelf of the convenience-store aisle in English. It can be a quarter-baguette with butter and ham, a hunk of pain de campagne with a slice of saucisson, a leftover heel of bread with whatever cheese was in the fridge, or a buttered tartine with jam at four in the afternoon. The category is defined by use rather than by recipe: the casse-croûte is what you eat when you need food fast and the meal will come later.
The structural logic is the structural logic of bread that was already in the kitchen. A casse-croûte is rarely made with a fresh-from-the-oven baguette, because if you had a fresh baguette you would be making a proper sandwich. The bread is yesterday's, sometimes the day-before-yesterday's, sometimes a heel from a loaf that has spent the morning getting a little dry. The filling is whatever requires minimal preparation: butter, a slice of charcuterie, a wedge of cheese, an end of leftover roast. The technique is to assemble it in under a minute, eat it standing up, and not photograph it. It is the antithesis of the apéro tray and the cousin of the worker's lunch.
The variations are organized by region and by occupation rather than by ingredient. The miner's casse-croûte in the Nord was historically a thick slab of pain de campagne with lard and onion, eaten cold underground. The vineyard worker's casse-croûte in Burgundy is the same bread with rillettes and a glass of red. The schoolchild's quatre heures, the four o'clock snack, is a piece of baguette with a square of dark chocolate or a smear of butter and jam. The Casse-Croûte Ouvrier, the working-class hearty version, gets its own entry. The broader Pain Garni family covers the wider non-baguette bread traditions on which the casse-croûte draws when the kitchen has whatever else is around. The whole category is a reminder that the French sandwich, before it was a Parisian boulangerie product, was the food you made with bread that needed using up.