Two world cities face off, and only one question matters: does the city on the right have a higher or a lower population than the one on the left? Guess right and it slides over to become the new benchmark; guess wrong and the run is over. There's no fixed round length — the streak just keeps going for as long as you keep being right.

The pool covers 400 of the world's best-known cities, from Tokyo's 37 million all the way down to Reykjavík's 140 thousand, so a run swings between megacities and cosy capitals. It's equal parts geography knowledge and nerve — everyone knows Tokyo beats Oslo, but is Madrid bigger than Santiago?

How to play

• The left card shows a city with its population revealed; the right card shows the challenger.
• Tap Higher if you think the challenger's population is bigger, Lower if you think it's smaller.
• A correct guess reveals the number, slides the challenger to the left, and deals a new city — your streak climbs by one.
• One wrong guess ends the run. Your score is the streak you reached.
• Your best streak is shown in the top bar — beat it and the new record sticks (and counts on the leaderboard).
• Get through all 400 cities without a miss and you've fully cleared the quiz at a streak of 399 — but the run doesn't stop there. The deck reshuffles and keeps dealing, so a cleared quiz just means the leaderboard counts it as perfect while your streak carries on.

What counts in this quiz

• Populations are urban-agglomeration estimates from the UN's World Urbanization Prospects (2023/2024 revisions), rounded — the whole metro area, not just the city proper.
• 400 cities across every inhabited continent, weighted toward places most players will recognise.
• If two cities are a statistical coin-flip, the deck avoids dealing them next to each other — and in a genuine tie, either answer counts as correct.

What's not included — and why

• City-proper counts are not used — "city limits" definitions vary too wildly between countries to compare fairly (Chongqing's municipality alone would outrank most metro areas).
• Micro-nations and towns under ~100k don't appear; the smallest entry is Reykjavík.

Good to know

• Tokyo has been the world's largest urban area since the 1950s, but Delhi is projected to overtake it before 2030.
• More than half of the cities in this quiz didn't exist in their current scale 100 years ago — Lagos alone has grown roughly 50-fold since 1950.
• The photos are Wikimedia Commons lead images of each city, credited on the card as you play.