Why round won
The round case is so dominant that it reads as the absence of a choice, but its supremacy is engineering before it is taste. A movement is round because its gear train is; a round case wastes no space around it. Threaded casebacks and bezels — the basis of real water resistance — want a circular sealing surface. Round cases machine efficiently from bar stock, and a round bezel can rotate. When the wristwatch had to become waterproof and mass-manufacturable in the twentieth century's middle decades, roundness was the path of least resistance, and everything else became, by definition, a statement.
That is the correct frame for case shapes: every non-round case is a deliberate argument against convenience — usually an argument about elegance, made at measurable cost in water resistance, manufacturing complexity, or movement fit. The great shaped watches are precisely the ones that make the cost feel worth it.
The rectangle and the square
The rectangular case is the great counter-tradition, born in the 1910s and reaching its full vocabulary in Art Deco's decade. Cartier's Tank (1917) fixed the type: brancard sides running into the strap, the case reading as architecture rather than instrument. The rectangle's logic is the wrist itself — an arm is longer than it is wide — and its natural evolution was the curved rectangle: the Tank Cintrée and the curvex watches of the 1930s, bent to lie along the arm's arc. The Jaeger-LeCoultre Reverso (1931) added the form's masterpiece of function, a rectangular case that slides and flips to protect its own crystal. Squares are the rectangle's blunter sibling — harder to wear, rarer to see done well, with the Tank Carrée and the square Monaco chronograph as the canonical exceptions. Rectangles demand shaped or small round movements, give up serious water resistance, and in exchange own the territory called "dress" more completely than any round watch can.
Tonneau, cushion, and the soft geometries
Between circle and rectangle lie the softened shapes. The tonneau — barrel-shaped, bowed sides, flattened ends — is a 1910s form that reads instantly as early-twentieth-century romance; Franck Muller built a modern house almost entirely on it, and Richard Mille's case, for all its technicality, is a tonneau. The cushion (coussin) — a square relaxed toward a circle — was the great case of the 1920s and the shape of the first Panerai prototypes; its modern revival rides the vintage-diver wave. The oval and ellipse belong mostly to the 1970s, with Patek's Golden Ellipse (1968, proportioned on the golden ratio) the form's one immortal. These shapes share a property: they keep enough roundness to house a round movement and seal tolerably, while signaling era and softness — which is why revivals of them always accompany nostalgia cycles.
The integrated-bracelet sports watches of the 1970s — Royal Oak's octagon (1972), Nautilus's rounded porthole (1976), the 222's notched circle — are usually discussed as bracelets, but they were equally a revolution in case thinking: bezel geometry as identity, case and bracelet designed as one object, steel finished like a precious metal. Genta's octagon proved a shape could be a brand. Nearly every "new" sports-luxury case since is a footnote to that decade.
The asymmetric and the driver's watches
The furthest shore is the case that abandons symmetry altogether. Drivers' watches of the 1930s–50s angled their dials so a hand on a steering wheel could read time without rotating the wrist. The 1950s–70s produced the great asymmetrics: Hamilton's electric Ventura (1957), the shielded "asymmetric" Omegas and Longines of the late 1960s, Patek's brutalist-leaning Gondolo experiments. Cartier kept a parallel tradition of the deformed classic — Crash (1967), the bent ovals and parallelograms of the London years — establishing that a great house could treat its own forms as material for surrealism. Asymmetric cases are the hardest to wear, the hardest to manufacture, and the most identity-dense per square millimetre; they are collected today precisely because no committee would approve them.
Choosing a shape: what it commits you to
Shape is the watch's loudest single signal, and it commits the owner to practical facts. Round: maximum versatility, maximum choice, full water resistance, anonymity available if wanted. Rectangular and shaped: dress-first wearing, modest or no water resistance, smaller movement (often with the seconds deleted), and a watch that is recognized across a room. Shaped watches also wear differently than their dimensions suggest — a 25mm-wide rectangle with a long curved case occupies the wrist like a much larger round watch — which is why shapes must be tried on, not specified from a chart. The reward for the commitment is character per millimetre that round watches need complications or provenance to match.
Round is the engineering answer; every other shape is a position taken against it, paid for in water resistance and convenience and redeemed in identity. That is why case shape is the fastest read of a watch's intent — and why the tank, the tonneau, the cushion and the crash keep returning whenever watchmaking remembers it is also a branch of design.