The register, annotated

Notes on India's PIN code system.

Short, well-researched reads on the history and mechanics behind the six digits — for anyone who's ever wondered why a PIN looks the way it does.

1972 INDIA POST
04 · 03 · 2026

How India's PIN Code System Began

Before computers touched a single envelope, one man's numbering scheme had to make manual sorting work at national scale. Here's the story of 1972.

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HO SO BO BO
11 · 03 · 2026

HO, SO, BO, and GPO — What Do They Actually Mean?

Every post office in the register carries a type stamp. Here's the hierarchy behind those four letters, and why it matters on a form.

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900xxx
18 · 03 · 2026

Why Some PIN Codes Start With 9: Inside the Army Postal Service

A 900000-series PIN doesn't map to any place on a map. Here's why, and how mail actually reaches soldiers serving anywhere in the world.

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110001–110097
25 · 03 · 2026

Why Delhi Has Such a Dense Cluster of PIN Codes

Delhi packs nearly a hundred distinct PINs into one metro area. The reason has more to do with administration than geography.

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500034
01 · 04 · 2026

How E-commerce and KYC Forms Use Your PIN Code

That instant "delivery in 2 days" estimate at checkout starts with your PIN. Here's everything it's quietly checking before you notice.

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172114
08 · 04 · 2026

The World's Highest Post Office Has a PIN Code Too

At 14,567 feet in Himachal Pradesh, mail still arrives — carried on foot, sorted under PIN 172114 like anywhere else.

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190001
15 · 04 · 2026

India's Only Floating Post Office Has Its Own PIN Code

Moored on Srinagar's Dal Lake since 2011 — a working post office you reach by shikara, not by street.

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1
22 · 04 · 2026

When One Building Gets Its Own PIN Code

Infosys's Mysore campus shares its PIN with no neighbour at all. Here's why India Post does this.

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