Why every house is three metres wide

Look down a Vietnamese street and the first thing that registers is the width of the buildings: three or four metres of frontage, five storeys up, painted a different colour from both neighbours and often a different height.

The shape has a name: nha ong, literally the tube house. It explains more of what a Vietnamese city looks like than any other single thing.

What are the proportions?

Narrow at the front and very long behind, in a ratio that holds across the whole country: about four metres of street frontage against twenty or thirty metres of depth.

The shape is not a quirk of the old quarters. It is what is still being built. New suburbs go up as rows of thin glass-fronted towers on the same 4-metre plots, which is why a Vietnamese street of new houses looks nothing like a Western housing estate.

Where does the shape come from?

Frontage was the thing that got measured. Taxation based on street width is documented under the Nguyen dynasty and again in the colonial period, and later land allocation kept plots narrow.

The tax story is true enough to repeat and too simple on its own. What kept the shape alive is that it works.

A narrow front on a busy street is a shop. The family lives behind and above it, and the motorbike parks in the front room at night. The street vendor guide describes the trade that happens on the pavement outside, and the two are the same economy. It is also why deliveries arrive on a motorbike rather than in a van.

What is inside?

A sequence of rooms rather than a floor plan, arranged front to back. The front room is a shop, a cafe, a repair business or a place to park two bikes. Behind it comes a living room with the ancestor altar — the religions guide explains why that altar is where it is.

A light well comes next, open to the sky, and it is the only way daylight reaches the middle of a 30-metre house. The kitchen and a small courtyard come next, and bedrooms stack above, connected by a staircase that gets steeper the higher it goes.

The staircase is why the older parents guide tells you to ask about lifts. A five-storey tube house with a family guesthouse on the third floor is a common booking and a hard climb.

What does it do to a street?

The plot line produces the look everyone photographs without knowing why. Each plot is owned and built separately, so no two facades match in height, colour or style, and the roofline goes up and down like a broken graph.

The same arrangement produces the ghost walls. When one house comes down, the shared walls on either side keep the outline of its staircase and its floors, painted in the neighbours' colours. Once you have noticed one you see them everywhere.

Where should you look at them?

Three places, in increasing order of preservation, and the third is any street you happen to be standing on. None of them needs a ticket except the preserved houses.

Hanoi's Old Quarter has the narrowest and oldest, some under 2 metres wide, and the preserved merchant house at 87 Ma May is open to visitors. Hoi An has the full merchant version with a courtyard and a shophouse front, and several charge a small entry fee — the Hoi An things to do page covers which.

Third is any provincial main road, where the new ones are going up right now in the same proportions. The French colonial architecture guide covers the other half of what a Vietnamese street is made of.

What is it like to stay in one?

More common than most visitors realise. A large share of family guesthouses and homestays in Vietnamese cities are tube houses, which means a narrow reception with a bike in it, a steep staircase, and a room with one window at the front or the back.

The middle rooms are the ones to avoid: on the light well, they get borrowed daylight and every sound in the building. The where to stay guide covers what to ask before booking, and a room facing the street is the answer here more often than not.