Cheese and tomato is a sandwich defined by a single engineering problem: the tomato bleeds. A ripe sliced tomato is mostly water held in a thin skin, and the moment it is cut and laid against bread it starts releasing that liquid into the crumb. Inside the gap between making this sandwich and eating it, the bread under the tomato can go from soft to sodden. Everything that matters about the build is a response to that bleed. The cheese is chosen and placed for flavour and as a barrier in equal measure, and the difference between a clean sandwich and a wet one is whether the maker took the tomato's water seriously.
The craft is containment before balance. The tomato is salted and left a few minutes so it weeps out its loose liquid before it goes into the sandwich rather than after, and the watery seed pulp can be scooped away so only firm flesh remains. A firm, sharp Cheddar cut as a thick continuous slab is laid directly against at least one face of the bread to act as a wall the tomato's remaining moisture cannot cross, which is why the cheese is sliced rather than grated here: a solid sheet defends, loose shreds do not. Butter spread hard to the edges on both slices is the second line of waterproofing and also the seasoning bridge, carrying salt across a pairing that is otherwise rich cheese against bland wet fruit. A grind of black pepper or a few torn basil leaves is the common lift, because tomato and Cheddar alone read flat without an aromatic note between them. The bread is soft and plain, and the sandwich is made as close to eating as possible, because no amount of butter beats a tomato given an hour.
The variations are mostly which other vegetable joins the wet one. Adding the full salad set turns it into a cheese salad sandwich and spreads the moisture problem across several fillings; a smear of salad cream or mayonnaise seasons the tomato directly and adds a tangy emulsion; raw onion drops in a sharp crunch against the soft slice. Each deserves its own article rather than being crowded in here.