The arch is the most fundamental aspect of architecture. Build nothing but an arch—it would still be instantly recognizable as architecture.

Pentagon "Here, take this concrete and build runways for our B-52 bombers to wipe out Vietnam."
King Savang Vatthana "I have a better idea."

The arch is a fantastic metaphor or symbol of faith, civilization, immortality, love. (Saint John Cassius)

"All building naturally divides into two classes, the architecture of the beam and that of the arch." — William Pitt Preble Longfellow, 1899

Beams vs. Arches is the original clash of worldviews, the Greek vs. the Roman. Western Civ. brought the two traditions together in harmony.

My favorite arch is the discharging arch, or the relieving arch: it does its work in silence, protecting openings and lintels from collapse.

Even Rafael Moneo, who only builds nonsensical modernist garbage, uses relieving arches to great effect in the National Museum of Roman Art.

The true—invisible, unsung—hero of archwork is the inverted arch, used in foundations to allow large buildings safe footing on soft ground.

Flat arches! Now here is an arch that looks physically impossible. Also knowns as a Dutch or French arch. For perfectly flat wall openings.

How they work is shown in this video (the arch is near the ground but it is the same principle in action):

Importantly, we trust arches. Humans and arches have a long history. That is why this brick veneer flat opening looks so jarring, so unreal:

A "fake arch" is the corbel arch, most famous example, the 13th c. B.C. Lion Gate. Tragedy is, all kids learn to build these nowadays: LEGO.

If you seriously want your child to understand building physics, architecture: avoid LEGO like the plague, give them these wooden blocks.

My second favorite arch: the diaphragm arch, first used in Roman Syria 2nd c. A.D. without which our great cathedrals would be impossible.

In many denser European towns the diaphragm arch was used to support the slender walls of freestanding buildings, usually made of brick.

For tourists, the most famous diaphragm arches are the Bohemian ones, called "prampouch" in Czech, adding tons of charm to the old towns.

Used cleverly diaphragm arches are easily & beautifully built into doorways, gatehouses and even turned into covered bridges between bldgs.

The modern alternative, reinforced concrete—short life, hugely wasteful, requires massive industrial infrastructure. Bring back the arch!