Your fintech needs to send $1,000 USD to a supplier in Singapore. Your Nigerian bank doesn't have a direct relationship with the Singaporean recipient bank. They use a CORRESPONDENT, a US bank where both your bank and the recipient bank hold accounts. Your bank debits its Nostro (the account it HOLDS at the US bank), the US bank credits the Singaporean bank's Vostro (the account they HOLD at the US bank). The wire passes through 3 institutional ledgers, each taking 15-50 USD in fees. This lesson walks the Nostro/Vostro pattern from your fintech's perspective so the ₦1,650 'mysterious wire fee' on the user's statement actually makes sense.