Stripe’s placeholder postcode is an industrial park just north of Birmingham.

I was filling in my credit card details on a website this evening:

I got curious about the placeholders. They seem to be consistently used across instances of this form; not specific to the single instance I was seeing.

Two of them seem straightforward, where somebody has used sequential digits to indicate a dummy value:

• 1234 1234 1234 1234 isn’t a real credit card number. The final character of valid credit card numbers is a check digit, calculated using Luhn’s algorithm, used to detect mistakes and typos. This card has the wrong check digit, so we know it isn’t real.

• 07400 123456 could be a real UK phone number. Ofcom publish numbering data, and every phone number starting 07400 belongs to “Hutchison 3G UK Ltd”, more commonly known as “Three”. They can choose which numbers to allocate within this range, and given it’s a popular placeholder in online forms, I wonder if this number is deliberately unused. (No, I didn’t dial it to find out.)

But what about the placeholder post code? What is WS11 1DB?

It’s a road in Cannock, just north-west of Birmingham. I thought maybe it would be a pseudo-address, like the building in Delaware which is the registered office of thousands of businesses – or perhaps the address of a Stripe office – but no, it looks like it’s just a regular postcode.

You can find local businesses that list it as their address, like an optician, a ceiling insulation firm, and a metal fabricator – nothing to do with online payments. (Most of the businesses at the postcode don’t even have an online presence except Companies House and listing aggregators.)

Occasionally the postcode appears in other places, but it’s only instructing people how to use a form. “Fill in your real postcode, not WS11 1DB.”

I can’t find any information about why this was chosen as the placeholder, and now I’m intensely curious. Does Stripe have a secret link to Cannock? Does this code provide some useful UI property? Was it A/B tested and found to be easily recognised as a placeholder?

If you know what’s going on, please get in touch.