St. Louis Skyline

The Gateway Arch is 630 feet tall, 630 feet wide at the base, and its cross-section is an equilateral triangle that tapers from 54 feet at the ground to 17 feet at the top. Eero Saarinen designed it as a weighted catenary — the curve a hanging chain makes, inverted — and that precise curve is why it looks effortless rather than merely tall.

It is also the single most difficult element in this entire catalogue to produce. A free-standing arch has no internal support while it prints, the span is wide relative to its thickness, and the taper means the cross-section changes along the whole length. Get the curve slightly wrong and the eye knows instantly, because this is a shape people have looked at their whole lives.

The Arch tends to dominate every description of St. Louis, but the skyline behind it does real work. One Metropolitan Square is the tallest building in Missouri at 593 feet, finishing in a sharply peaked roof. The Old Courthouse dome sits low and green directly within the Arch's span — a 19th-century building framed by a 20th-century monument, which is a genuinely unusual composition.

That framing is the part worth preserving. Shift the Arch a few millimetres in a model and the dome sits outside it, and the view stops being the one people recognise from the riverfront.

The Gateway Arch dominates everything: 630 feet tall, 630 feet wide at the base, a weighted catenary in stainless steel with a triangular cross-section that tapers from 54 feet to 17.

Behind and through it, One Metropolitan Square rises 593 feet to a sharply peaked roof as the tallest building in Missouri. The Old Courthouse's green dome sits low and directly inside the Arch's span, which is the composition the city is known for and the single alignment the model has to get exactly right. The Mississippi runs along the eastern edge in matte black.