The afternoon-tea tongue sandwich is pressed ox tongue put through the manners of the tea tray. The defining facts are subtractive: the crusts gone, the bread thin, the meat shaved rather than sliced, the whole thing trimmed square and cut into fingers small enough to take in two bites without a plate. Where the everyday tongue sandwich is an honest cold-cut lunch, this is the same meat made delicate on purpose, chosen for the tray precisely because tongue is mild, fine-grained, and pale, and so behaves itself among cucumber and smoked salmon rather than shouting over them. The trimming is the design, not a flourish: a crust would resist a tender filling and the form depends on nothing resisting.
The craft is in the thinness of the meat and the restraint of the seasoning. Tongue for the tea tray is shaved very thin across the grain so it folds and yields rather than slabbing, and it is laid in a single even layer so the finger presses flat and cuts cleanly without the meat dragging. The mustard, if it appears at all, is the gentlest possible smear, often softened into the butter rather than laid on as a stripe, because the point of the register is balance and delicacy and a sharp burst would break it. Butter spread to the edges waterproofs the crumb and bridges the brine's salt to the bread, so the finger survives the wait between assembly and the table. Plain soft bread is correct because it gives a tender filling no chew to fight.
There is little room to vary inside the form. A whisper of mustard cress, the thinnest watercress leaf, or mustard worked into the butter is near the limit before it stops being this sandwich. The everyday tongue sandwich, thicker-cut and frankly mustarded at the counter, is the other reading and is treated on its own. Each deserves its own article rather than being crowded in here.