19
19 is a prime number, the atomic number of potassium, and forms a twin-prime pair with 17.
- Odd
- Prime
Where 19 fits
Why 19 is special
19's mathematics (prime, twin with 17) is ordinary by design — its best-known everyday reference is jokingly practical: 'the 19th hole' golfing idiom for the clubhouse bar.
Same number, different meanings
How 19 is understood across different contexts — never implying one meaning because of another.
- Mathematics
19's mathematics (prime, twin with 17) is ordinary by design — its best-known everyday reference is jokingly practical: 'the 19th hole' golfing idiom for the clubhouse bar.
- History
Nineteen years is almost exactly 235 lunar months — the two run out of step by about two hours — which is why the figure anchors calendars that must track both Sun and Moon. Named for the Athenian astronomer Meton, who described it in the 5th century BCE, the cycle sets the pattern of leap months in the Hebrew calendar and underlies the computus, the calculation that fixes the date of Easter.
- Science
Comet 19P/Borrelly was imaged by Deep Space 1 in 2001, showing an elongated nucleus among the darkest surfaces measured in the solar system.
- Law & regulation
The number of the amendment barring denial of the vote on grounds of sex is 19 (United States, ratified 1920). It removed sex as a permitted ground for denying the vote; it did not confer the vote, and other barriers continued to exclude many women, particularly Black women in the South, for decades afterwards.
- Sport
'The 19th hole' is a well-known golfing idiom for the clubhouse bar, humorously extending a standard 18-hole round by one more notional 'hole'.
- Religion & belief
In the Bahá'í Faith, the calendar is built of nineteen months of nineteen days. Three hundred and sixty-one days, with intercalary days inserted before the final month to align with the solar year. The number derives from the Bábí movement that preceded the faith, where it counts the Báb and his first eighteen disciples. This is a scholarly account rather than the tradition's own framing.
Factors & digit properties
- Divisors (2)
- 1, 19
- Prime factorisation
- 19
- Digit sum
- 10
- Digital root
- 1
Representations
- Decimal
- 19
- Scientific notation
- 1.900 × 10^1
Roman numerals
X I X
XIX uses the subtractive pair IX: one before ten — ten minus one.
Binary
1 0 0 1 1
1 × 16 + 0 × 8 + 0 × 4 + 1 × 2 + 1 × 1 = 19
Octal
2 3
2 × 8 + 3 × 1 = 19
Hexadecimal
1 3
1 × 16 + 3 × 1 = 19
Read as powers of two: 16 + 2 + 1 = 19.
In historical numeral systems
Egyptian
1 hobble, 9 strokes
1 × 10 + 9 × 1 = 19. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
1 corner wedge + 9 vertical wedges
19 fits in a single sexagesimal place: 1 corner wedge + 9 vertical wedges.
Greek (Ionic)
iota, then theta, marked as a numeral
iota-theta + keraia
iota + theta written in descending order and added — the letters carry their values wherever they stand, so this is addition, not place value.
Hebrew numerals
yod, then tet
yod-tet
yod (10) + tet (9) = 19, written in descending order.
Chinese numerals
shí jiǔ
shí jiǔ
十 is ten. Adding 九 after it gives 19 — "ten jiǔ". Writing 一十 is not wrong, but nobody writes eleven that way on its own. The financial (anti-fraud) form is 壹拾玖.
Maya
3 bars + 4 dots
19 fits in the bottom place: 3 bars + 4 dots, since a bar is five and a dot is one.
Systems that simply cannot reach this value are left out. Where a script needs a font most readers do not have, the reading is given instead of the glyphs — see number systems for each system in full.
Other mathematical patterns
- Deficient number
- Happy number
- Twin prime
Related numbers & connections
19 is a factor of 38 (19 × 2 = 38).
View 38 →18 (a standard golf round) and 19 (the 'clubhouse' 19th hole) are consecutive integers connected by the same golfing idiom.
View 18 →17 and 19 are twin primes — two primes exactly two apart.
View 17 →Mathematically related
Nearby in a sequence
- is adjacent to18
why?
Strength 0.50, confidence "high".
Shares a distinctive class
- is associated with17
why?
Strength 0.50, confidence "high".
Numbers Around Us
- appears inLaw and public systems
why?
Strength 0.50, confidence "high".
- appears inCulture and belief
why?
Strength 0.50, confidence "high".
- appears inStandard golf round (18 holes)
why?
Strength 0.50, confidence "high".
Other connections
- is associated with9969
why?
Strength 0.50, confidence "high".
History
Nineteen years is almost exactly 235 lunar months — the two run out of step by about two hours — which is why the figure anchors calendars that must track both Sun and Moon. Named for the Athenian astronomer Meton, who described it in the 5th century BCE, the cycle sets the pattern of leap months in the Hebrew calendar and underlies the computus, the calculation that fixes the date of Easter.
DocumentedHistoryPeriod: 5th century BCE onwards
Science
Comet 19P/Borrelly was imaged by Deep Space 1 in 2001, showing an elongated nucleus among the darkest surfaces measured in the solar system.
DocumentedSciencePeriod: 1904–19 is the atomic number of potassium. Potassium ions work with sodium to fire every nerve impulse and heartbeat, and potassium salts are the basis of most agricultural fertiliser.
DocumentedScience
Law & regulation
The number of the amendment barring denial of the vote on grounds of sex is 19 (United States, ratified 1920). It removed sex as a permitted ground for denying the vote; it did not confer the vote, and other barriers continued to exclude many women, particularly Black women in the South, for decades afterwards.
DocumentedLaw & regulationGeography: United States, ratified 1920
Mathematics
19 is the third centred hexagonal number — one cell surrounded by rings of 6 and 12 — after 1 and 7. That is why 19 recurs in things built from bundled circles: 19-strand wire rope, 19-fibre optical bundles and 19-cell hexagonal puzzle grids all use it because a hexagon of side 3 packs exactly that many.
DocumentedMathematics
Sport
'The 19th hole' is a well-known golfing idiom for the clubhouse bar, humorously extending a standard 18-hole round by one more notional 'hole'.
Widely documentedSport
Religion & belief
In the Bahá'í Faith, the calendar is built of nineteen months of nineteen days. Three hundred and sixty-one days, with intercalary days inserted before the final month to align with the solar year. The number derives from the Bábí movement that preceded the faith, where it counts the Báb and his first eighteen disciples. This is a scholarly account rather than the tradition's own framing.
Tradition / practiceReligion & beliefCulture: the Bahá'í FaithPresented as belief or tradition, not as an established fact.
Common questions
What is the 19th hole?
The clubhouse bar. A round of golf is 18 holes, so the joke extends it by one more — and the phrase is old and standard enough that many clubs simply label the bar that.
Why does 19 matter to calendars?
Nineteen years is almost exactly 235 lunar months, out by roughly two hours. That near-coincidence, known as the Metonic cycle, is how lunisolar calendars stay in step with both Sun and Moon: the Hebrew calendar inserts a leap month seven times in each 19-year cycle, and the calculation that fixes Easter is built on the same period.
Why does 19 turn up in wire rope and fibre bundles?
Because a hexagon of side 3 holds exactly 19 circles: one in the middle, then rings of 6 and 12. It is the third centred hexagonal number, and hexagonal packing is the tightest way to bundle round things — which is why 19-strand rope and 19-fibre optical bundles are standard sizes.
Is 19 a prime number?
Yes, 19 is prime — it has exactly two positive divisors: 1 and itself.
What are the factors of 19?
The factors of 19 are 1, 19.
What is 19 in Roman numerals?
19 in Roman numerals is XIX. IX is subtractive: one before ten — ten minus one. Written additively it would be XVIIII, which Roman inscriptions used freely — the strict subtractive form is a later convention.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 7 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026