The BLT is three ingredients on toast, and the one that defines it is the toast. Bacon, lettuce, and tomato on soft bread is a salad that falls apart in the hand; the same three on toast is a sandwich, because toasting is the only thing standing between a ripe tomato's water and structural collapse. Bacon, lettuce, tomato, and a layer of mayonnaise on toasted bread is the entire build, and the genius of it is how little it is. There is no cheese to hide behind and no sauce to carry it. The sandwich is judged on the rendering of the bacon, the ripeness of the tomato, and whether the toast holds, and nothing else is offered as cover.
It works because each element is doing one job and the build is arranged around the tomato's threat. The bacon is the flavor and the structure: salty, rendered crisp, and rigid enough to give the soft sandwich a little spine, and it is laid in a flat overlapping layer so every bite gets some rather than pulling the whole strip out in one tug. The tomato is the reason the whole thing is precarious, a ripe slice shedding water the moment it is cut, so it is salted to season it and the mayonnaise is spread edge to edge on both inner faces as a moisture barrier between the wet slice and the toast. The lettuce is the cold crunch that the bacon and tomato lack, and it is positioned between the tomato and the bottom slice so it doubles as a second shield for the bread. The toast has to be crisp and recent, and the sandwich is built and cut fast, because a BLT is a sandwich on a clock: the longer it sits, the further the tomato's water travels into the crumb, and a soggy BLT is a failed one. The diagonal cut is not only for looks; it gives a shorter, more stable bite into a sandwich with no binder holding the layers. Timing is the craft. There is no technique to rescue a BLT made too far ahead.
The variations stay inside the same fragile, three-part frame and each deserves its own article rather than being crowded in here. Add a fried egg and it becomes a heartier breakfast-leaning build; swap the tomato for a fried green one and the acid and crunch change character entirely; add avocado for a BLAT and the richness shifts. The club sandwich is the BLT's direct descendant, the same base braced with a third slice of toast and a layer of poultry to solve the instability the BLT lives with on purpose. Each of those is a single move on a sandwich that barely has parts to move, which is exactly why each earned a name of its own.