What are nostro and vostro accounts?
Your money never crosses the border. It was already there.
The message crosses the border. The money never does. It was sitting in Germany before you decided to send anything.
The sentence that unlocks everything
When you send money from India to Germany, no rupees travel to Germany. No euros travel to India. Nothing moves.
What happens instead is that two banks that already hold accounts with each other adjust the numbers in those accounts. Your bank reduces what it holds; the German bank increases what it owes its customer. The money was already in Germany. It had been sitting there, waiting, held by your bank, before you ever decided to send anything.
Nostro is Latin for "ours": our account, held at their bank. Vostro is "yours": your account, held at our bank. Same account, two viewpoints, depending on who is doing the bookkeeping.
Why this shapes everything else
Once you see it, three mysteries dissolve at once.
Why some corridors are cheap and others are brutal. A corridor is cheap when your bank already holds a well-funded nostro in that currency. It is expensive when your bank has to route through two or three intermediaries, each of whom holds an account with the next, and each of whom takes a slice for the privilege.
Why transfers have cut-off times. These accounts are settled through domestic clearing systems that keep office hours. Miss the window and your instruction waits, not because anyone is lazy, but because the local rail is closed.
Why banks hate holding them. Every nostro account is money sitting idle in a foreign country, earning very little, tying up capital, and requiring compliance oversight. Banks have been closing them for a decade. Fewer accounts means longer chains, and longer chains means higher costs, which is the quiet reason cross-border pricing has not fallen the way domestic pricing has.
Two shopkeepers on opposite sides of a river, each holding a float of the other's currency in a drawer behind the counter. When a customer wants to pay across the water, nothing goes over the bridge except a note saying how much to move between the two drawers. The drawers were filled months ago, on a quiet afternoon, by people nobody thanks.
Where you meet it
Every international transfer you have ever made, and every fee you did not understand. When a transfer takes three days for no visible reason, you are watching a chain of these accounts settle in sequence, each on its own local clock.