The Maryland crab cake sandwich is defined by how little binds it together. The cake is mostly lump blue crab meat held with just enough egg, mayonnaise, and a few crumbs to keep it from falling apart, and the entire craft is the restraint of that ratio. Where a fried fish sandwich is a coating problem, this is the opposite: the goal is a patty that is barely a patty, sweet crab in large intact lumps with almost nothing standing between them, so the sandwich tastes of the Chesapeake and not of filler.
The craft is in the bind and the heat. Too much egg or breadcrumb and the cake turns to a dense dredge cake that could be anything; too little and it collapses in the pan or on the bun. The mix is folded, not stirred, so the lump meat stays in pieces rather than shredding into paste, and it is seasoned lightly, usually with an Old Bay-style blend and a little mustard, so the crab leads. It is pan-fried or broiled rather than deep-fried, because a delicate, loosely bound cake will not survive submersion and does not want a hard shell anyway; the surface is set just enough to lift cleanly and the inside stays soft and steamy. The bun is plain and soft, often a squishy roll or even a saltine-lined plate in the purist version, chosen so it cannot fight a fragile cake, and it is dressed cool: tartar or a similar bound sauce for acid and fat, a leaf of lettuce, sometimes tomato. A crab house works these to order, cakes formed loose and held cold, dropped to the heat only on the ticket so they reach the bun hot and intact.
The variations stay close to the cake. The broiled version skips the pan for a lighter, drier surface; the fried version accepts more binder so it can take the oil; the open-face build serves the cake on a single slice under the sauce and skips the top entirely. These are small turns on a sandwich whose whole identity is minimal binding, and the wider fried-fish shelf, the chain filet, the regional grouper and walleye and catfish, is solving a different problem and deserves its own articles rather than being crowded in here.