Evelyn Lamb has a delightful page-a-day calendar. Today (yes, a few days late) I learned that TWENTY NINE is the only word in English that is written with a number of straight-line strokes equal to its value. (This is in a sans serif font; in particular I is one stroke, not three.)
English is surprisingly rich in numbers that have all straight-line strokes. In the Latin alphabet the letters that are all straight lines are A, E, F, H, I, K, L, M, N, T, V, W, X, Y, Z. That leads to the following words that are all straight-lined and appear in numbers, and the number of straight lines that make them up:
– having more strokes than their value: FIVE (10), NINE (11), ELEVEN (19), TWELVE (18), FIFTEEN (20), NINETEEN (23)
– having less strokes than their value: TEN (9), TWENTY (18), FIFTY (12), NINETY (16)
Any word that equals its own value must combine elements from both these lists, and it’s not hard to see that TWENTY NINE is the only one that works. The full list of straight-line numbers in English is: 5, 9, 10, 11, 12, 15, 19, 25, 29, 55, 59, 95, 99. (All the larger numbers include “HUNDRED” or “THOUSAND” or something ending in “-ION”, so we can stop there.)
Evelyn suggests this as part of how to memorize the largest known prime (it’s a Mersenne prime, and she suggests doing it in binary so every bit is 1, so the hard part is remembering where you are).
It’s hard to even find straight-line numbers in other languages, because a lot of the alphabet is missing. They include:
- German ZWEI (2), ZEHN (10), ELF (11)
- Dutch EEN (1), TWEE (2), ZEVEN (7), TIEN (10), ELF (11) (edited 9/3: also ZWAALF (12))
- Norwegian has a lot: EN (1), FEM (5), ATTE (8), NI (9), TI (10), ELLEVE (11), FEMTEN (15), ATTEN (18), NITTEN (19), FEMTI (50), ATTI (80), NITTI (90) (and also 55 = FEMTIFEM, 58, 59, 85, 88, 89, 95, 98, 99)
- As does Danish: EN, FEM, NI, TI, ELLEVE, FEMTEN, ATTEN, NITTEN, with the same meanings as in Norwegian, but then the bigger numbers are formed irregularly.
- Swedish has less: EN (1), FEM (5), ATTA (8), ELVA (11) – the numbers are very similar to Norwegian but the “-teen” ending is “-ton”, not “-ti” like Norwegian.
- Spanish VEINTE (20), MIL (1000), and MIL VEINTE (1020)
- Italian VENTI (20), MILLE (1000), and MILLEVENTI (1020)
- Portuguese VINTE (20), MIL (1000) and MIL E VINTE (1020)
- French MILLE (1000), but not VINGT (20)
ELVA (Swedish for 11) is the only other one I could find that also has the self-referential property, and the Chinese numerals 一, 二, 三 if you want to stray from the alphabetic world. (Edited 9/3: also Dutch TIEN = 10, which I inexplicably missed before.)
(This post was edited to add the list of numbers and to clarify that ELVA is not the only straight-line number outside of English, but the only one I could find with this self-referential property.)