That’s right – according to the 2011 census, 154,233 people live in CR0. That’s around 35,000 more people than the next most populous – LE2 in Leicester – and an incredible six times bigger than the average population per postcode (24,713).

It’s the reason why we at the Advertiser constantly get (poorly researched) press releases saying Croydon has the most of ‘x’ or ‘y’ in the country (their market research always fails to take into account the huge population of CR0).

Out of interest, the least populous postcode is TR25 – which is for the people of St Martin’s, one of the Isles of Scilly.

But, back to the point, the answer lies – as so often with these things – in a quirk of history, since Croydon was among the first places in the country to get a postcode under the modern system.

CRO – note that’s the letter ‘o’ – arrived when the Post Office started rolling out a modern system which had been trialled in Norwich to the rest of the country, the point being that the codes would make it easier for the mail to be sorted.

It’s pretty huge when you see it on a map

So on May 1, 1967 Croydon got its postcode. As it was early days for the postcode it encompassed a large geographic area, from Thornton Heath in the northwest to New Addington in the southeast. Further to that, the borough’s population has boomed by an extra 60,000 people since the introduction of postcodes, inflating the number of people in CR0.

Eventually most postcode areas when they were introduced were smaller, as it made it easier to sort the post.

Altogether, the CR0 postcode encompassed (and still does) Croydon town centre, Addiscombe, Selhurst, Broad Green, Shirley, Addington, New Addington, Coombe, Forestdale, Waddon, Beddington (which isn’t even in the borough of Croydon!) as well as parts of Thornton Heath and Woodside.

As an aside, Tony Benn, grandfather to former Croydon Councillor and 2015 Croydon South general election candidate Emily Benn, held the government position of Postmaster General when the rollout of postcodes was announced.

Note that Croydon’s first postcode was initially set up with the letter ‘o’ rather than a zero – the code simply being short for Croydon. But as the postcode system became more standardised, the system of two letters followed by a number was introduced, so CRO was tweaked to CR0 in 1974, when the rollout of postcodes was pretty much completed.

This also means that Croydon is the only postal district in the country where the chief district – where the head post office is based – ends in 0, rather than 1. In recent years, the CR0 postcode has also been used – as a portmanteau with the Bronx – for the town’s tongue-in-cheek nickname the Cronx, after which the the local brewery is named.

Another historical quirk of the early introduction of postcodes in Croydon means that the CR9, or sometimes CR90 postcode was given to high-volume postal addresses, usually businesses – and is not related to geography like the borough’s other postcodes.

And before all this makes you think that having a CR0 postcode may just be the best thing since sliced bread, a group of Beddington residents started a petition in 2009 to get themselves removed from the CR0 area, pointing out they were in fact from Sutton.

One of them said: “You live where you want to identify with and if you’re postcode doesn’t match, it’s wrong. This is a nice area, not like Croydon at all.”

Charming!