The World Solves
If you have only ever solved in English, you'll assume a crossword has a numbered grid and a list of clues printed beside it. Most of the world does not agree.
The arrow-word won
Across Germany, Russia, Poland, Brazil, Spain, Israel, the Netherlands and the Nordic countries, the standard puzzle puts the clues inside the grid, in the cells themselves, with a little arrow pointing at where the answer starts. No numbering. No separate list. Everything you need is in one square of newsprint.
Britain, America and English-language India kept the numbered list, and so did Italy and France - Latin-script print cultures that had built their own crossword traditions in the 1920s before anything else arrived.
The pattern underneath is hard to miss: the numbered list survived where the crossword was native to the language of power, and the arrow-word took everything else.
Two pieces of evidence make this more than a hunch. In Poland, the in-grid form outsells the numbered form roughly two to one. And in Germany, the ordinary word for the puzzle is Schwedenrätsel - the Swedish riddle - which German usage describes as "the Swedish puzzle known as the normal crossword". The imported form became the default so completely that the original now needs a qualifier.
(A word of caution: two different men are commonly named as the arrow-word's inventor, and neither claim stands up. Nobody knows who did it.)
1924, twice, in exile
The best coincidence in crossword history happened in 1924, and almost nobody tells it.
The first Russian crossword was written by Vladimir Nabokov, in Berlin. The first Hebrew crossword was written by Perski, in New York. Same year. Both by exiles. Neither published in the country whose language it was written for.
Both men faced the same problem before they could set a single clue: their language had no word for the thing. So they made one. Nabokov coined крестословица. The Hebrew is תשבץ, tashbetz - a word taken from the setting of stones in the priestly breastplate, which is to say from the idea of things fitted precisely into a frame.
That's a better metaphor than "crossword", and it points at something the languages kept arriving at independently. Swedish calls it ordfläta - a word-plait. Hebrew describes the structure as shti va-erev, warp and weft. Russian reaches for interlaced words; Polish for the crossing itself. Weaving, over and over, invented separately in half a dozen places.
Two exiles invented the crossword twice in the same year, in two languages that had no word for it, and both had to coin one.
The rule that was already old
Chinese lantern riddles - a tradition centuries older than any crossword - long ago codified a rule of composition: the characters of the answer must not appear in the riddle.
That's Afrit's principle. It's the cardinal rule of the cryptic clue, the thing every British setter is taught, arrived at independently, hundreds of years earlier, in a writing system with no letters to conceal. The problem is apparently not English at all. It's what happens to any language once people start hiding answers inside questions for sport.
Why sudoku travelled and the crossword did not
The crossword is stuck inside its language. A brilliant clue is untranslatable by construction; translate the answer and the wordplay dies.
Maki Kaji, founder of the Japanese publisher Nikoli, said so plainly: the same sudoku puzzle can be used in any country in the world, while for crosswords this would be impossible. The World Puzzle Federation builds its competitions on exactly that principle - puzzles playable regardless of language or culture. It's why a logic puzzle invented in Japan can become a British commuter habit within a year, and why no crossword has ever crossed a language border intact.
There's a footnote. Nikoli's puzzles were handmade, and it was proud of it - the house line was that you will never create Nikoli's sudoku puzzles. It was right, and it didn't matter. Computer generators flooded the market it had created and undercut it on price.
A French footnote
Georges Perec set more than three hundred grids, and wrote a three-hundred-page novel without using the letter E. He didn't treat the constraint as the price of the art. He treated it as the material.