Cheese and ham is the sandwich where the interesting question is the ratio. Two strong, salty, fatty ingredients are put between bread with nothing to mediate them, so the build is not really about whether they go together, they plainly do, but about which one leads. A thick slab of Cheddar with a single thin slice of ham is a cheese sandwich with a savoury accent. A generous fold of ham under a thin shaving of cheese is a ham sandwich with a dairy edge. The two are different sandwiches wearing the same name, and the cook decides which one arrives by how the layers are weighted.
The craft is balancing two ingredients that are pulling in the same direction. Both the Cheddar and the ham bring salt and fat, so a heavy hand on both at once produces a sandwich that is rich and one-dimensional, salt stacked on salt with no relief. The usual correction is to keep the proportions honest, cheese sliced with presence but not slabbed, ham folded rather than piled, so neither buries the other, and to let the bread and butter carry rather than compete. Butter to the edges bridges the salt across the slice and waterproofs the crumb. The ham is folded, not laid flat, so it has loft and the bite has give instead of a dense compressed plug. Soft plain bread is the carrier because the two fillings are loud enough on their own.
A single sharp counter changes the sandwich entirely, and each addition has its own logic. Mustard cuts the fat with heat; pickle or chutney answers it with acid and sweetness; tomato adds water and a sour note; toasting and pressing the whole thing turns it into a different sandwich again. Each deserves its own article rather than being crowded in here.