The teriyaki chicken sando is the comfortable middle of the Japanese chicken-sandwich spectrum, sweet where the fried versions are crisp and easygoing where they are loud. The filling is chicken cooked in or glazed with teriyaki, the lacquered sweet-soy sauce reduced until it clings, then layered onto soft shokupan with mayonnaise and lettuce. There is no breading and no fry; the chicken is grilled or pan-cooked, so the sandwich runs juicy and glossy rather than crunchy. What carries it is the interplay of the dark caramelized sweetness of the glaze, the cool tang of the mayonnaise, and the watery crispness of the lettuce, all wrapped in cottony bread. It is a sandwich most people in Japan have eaten from a convenience-store shelf without giving it a second thought, and its reliability is the appeal.
The craft is in the glaze and in the moisture. Teriyaki is soy sauce, mirin, sake, and sugar reduced to a syrupy coat; the chicken, usually thigh for juiciness and forgiveness, is cooked through and then glazed so the sauce caramelizes onto the surface without burning to bitterness or staying thin and watery. The key risk is wetness: an underreduced glaze plus juicy chicken plus mayonnaise plus lettuce is a lot of liquid for soft bread to hold, so the sauce has to be tight and the chicken rested or drained before it is built. The shokupan is trimmed and soft, the mayonnaise spread on the bread to add richness and a sharp counterpoint to the sweet glaze, the lettuce crisp and dry for lift. A good one is balanced: the teriyaki sweet and savory but not cloying, the chicken moist, the mayo cutting through, the lettuce fresh, the bread intact. A sloppy one collapses on sugar and water: a glaze so sweet it reads as candy, chicken dry from overcooking to compensate for a thin sauce, a soggy base where the glaze and mayo soaked straight through, or limp lettuce that adds nothing. The bind is the glaze plus mayonnaise; the sauce has to be reduced enough to grip the chicken and not run into the crumb, or the whole sandwich slumps.
That places it among the unfried chicken builds rather than the katsu and karaage line, a sandwich about a sauce rather than a coating. The variations move along the glaze and the additions: some push the teriyaki darker and more savory with extra soy, some sweeten and thicken it toward a near-candied lacquer, some add a slice of cheese or a layer of mayonnaise spiked with karashi, and some grill the chicken hard for char against the sweetness. Each of those tunes the sauce-and-chicken balance in its own direction and deserves its own article rather than being crowded in here.