Fear of the Number 17

Where it operates is Italy and the other countries with Greek and Latin roots, in which 17 counts as an ill-fated number for much the same sort of reason that 13 is avoided across the Anglo-Saxon world. What runs it is superstition, not the clinic.

Which older reasons made the number unlucky

Three older reasons are on record:

• in Ancient Greece the followers of Pythagoras despised it, on the grounds that it falls between 16 and 18, which are perfect representations of a four by four and a three by six quadrilateral respectively, so seventeen is what sits between two tidy things
• the Genesis flood narrative places the start of the flood on the 17th day of the second month
• in the Neapolitan smorfia, the traditional system assigning symbolic meanings to numbers for use in the lottery, 17 stands for 'a disgrazzia, misfortune

How Friday the seventeenth became Italy's unlucky date

In Italy the unlucky date is Friday the 17th, sometimes written friggaheptadekaphobia or paraskevidekaeftaphobia, and it works by stacking two negatives: Friday, from Good Friday and the death of Jesus, and the number itself. It arrives at least once a year and can occur up to three times. February, March and November all carried one in 2017 and again in 2023; January, April and July did in 2020. There were two in 2015 and in 2021, and there will be two in each year from 2025 through 2028. Five recent years supplied only one apiece: 2016, then 2018 and 2019, then 2022 and 2024.

One rule generates the whole pattern. Start a month on a Wednesday and its seventeenth lands on a Friday; start it on anything else and it does not.

Other countries put the same dread on other dates. Anglo-Saxon countries use Friday the 13th; Spain, Greece and South America use Tuesday the 13th.

In film the theme surfaces as The Virtuous Bigamist, whose Italian title is "Era di venerdì 17", and as "Shriek If You Know What I Did Last Friday the 13th", released in Italian as "Shriek - Hai impegni per venerdì 17?", where the number changes to match the audience.

Why the Pythagoreans disliked the number

The oldest reason on record is not about death at all. The followers of Pythagoras despised seventeen because of where it sits: between sixteen and eighteen, which are the perfect quadrilaterals four by four and three by six. Seventeen is the gap between two well-made rectangles, and being prime it makes none of its own.

That is an objection to a number for failing to be a shape, and it belongs to a school that treated number and shape as the same subject. Nothing in the modern superstition preserves the reasoning; what survived is the verdict.

The Genesis flood supplies a second early association: the water began on the seventeenth day of the second month.

What the number means in the Neapolitan smorfia

The Neapolitan smorfia is a traditional system assigning a symbolic meaning to every number for use in the lottery, and it gives seventeen as 'a disgrazzia, misfortune.

That is the superstition operating as infrastructure rather than as private dread. A player choosing numbers is consulting a published table in which seventeen already means what it means, and the meaning is fixed before anybody feels anything about it.

How often Friday the seventeenth actually happens

Friday the seventeenth, which has names of its own in friggaheptadekaphobia and paraskevidekaeftaphobia, requires a Friday to fall on the seventeenth of a month. That happens at least once in every year and can happen three times.

The distribution over a recent decade is uneven in a way that superstition does not accommodate:

• three in 2017 and 2023, in February, March and November, and three in 2020, in January, April and July
• two in 2015 and 2021, and two in each year from 2025 to 2028
• one apiece in 2016, 2018, 2019, 2022 and 2024

A dreaded date whose frequency triples and halves according to which weekday the first of January lands on is a fact about the Gregorian calendar rather than about the number.

Which date other countries dread instead

Friday the seventeenth is the Italian version of a form that exists in several places and agrees on almost nothing. Anglo-Saxon countries put the unlucky conjunction on Friday the thirteenth. Spain, Greece and South America put it on Tuesday the thirteenth.

Three cultures, two numbers, two weekdays, and one structure: a day of the week already carrying a bad association, multiplied by a number already carrying one. What is stable across all of them is the multiplication, not either of the things being multiplied.