Communication · c. 550 BCE

Persian royal post

Relays of mounted couriers carry royal messages across an empire in days, not months.

The Achaemenid Persians ran relay stations along the Royal Road, about 2,700 km from Sardis to Susa, where fresh horses and riders waited. Herodotus reported that messages crossed it in about nine days versus ninety on foot, the model for every later state postal system.

See it in the living tree Wikipedia

Arcs above join it to what it stood on, arcs below to what it led to. The bar places this stretch on the tree's whole timeline.

What it built on 2

Writing (cuneiform) 3200 BCE
Livestock domestication 8500 BCE

What it led to 1

Cursus publicus 20 BCE

Related measurements

The dashed line marks when it first worked.

Letters and parcels handled in the UK Latest: 16.7B items per year (1993) Source: Our World in Data