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If you’ve ever looked at a banking system and felt lost the moment someone said “debit” and “credit”, this page is for you. You don’t need an accounting background. You just need one idea — and once it clicks, almost everything else in core banking falls into place. Here it is: money never appears out of nowhere, and it never simply vanishes. It moves. Every time money moves, it leaves one place and arrives at another. Recording both sides of that movement is the whole idea behind double-entry accounting.

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Money always comes from somewhere

Think about pouring water between two buckets. The water that leaves the first bucket is exactly the water that fills the second. Nothing is created, nothing is lost — it just changes place. You can always check that the amounts match. A core banking system records money the same way. Every movement has two parts:
  • Where it came from — the source
  • Where it went — the destination
If a customer sends R$100 to a merchant, the system doesn’t just say “the merchant has R$100 more”. It records the full story: R$100 left the customer and R$100 arrived at the merchant. Two sides of one movement, always equal.
This is why it’s called double-entry: every movement is written down twice — once as it leaves, once as it arrives. A single-sided record would let money quietly appear or disappear. Double-entry makes that impossible.

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The two sides have names: debit and credit

The two sides of a movement have traditional names. Don’t let them intimidate you — they’re just labels for “out of” and “into”:
  • A debit is the entry on the account that money moves out of.
  • A credit is the entry on the account that money moves into.
That’s it. When the customer pays the merchant, the system records a debit on the customer’s side and a credit on the merchant’s side. Same amount, opposite directions. You’ll sometimes see debit and credit described as “decrease” and “increase”, but that’s not always true — it depends on the kind of account. The reliable way to think about it is always the same: debit is the source, credit is the destination.

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Why both sides are always required

Because every movement is recorded on both sides, the totals always have to match. The sum of everything that left equals the sum of everything that arrived. If they don’t balance, something is wrong — and the system won’t let the movement through. This single rule — the two sides must balance — is what makes a financial system trustworthy. It means the records can never silently lose money or invent it. Every value in the system can be traced back to a movement that has a clear source and a clear destination.
Each individual part of a movement — a single debit or a single credit — is called an operation. A movement of money is built from operations that balance out. You’ll see this word a lot, so it’s worth remembering: an operation is one part of a movement.

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In short

  • Money doesn’t appear or disappear — it moves from a source to a destination.
  • Double-entry means every movement is recorded on both sides: a debit (out of) and a credit (into).
  • The two sides always carry the same amount, so the books always balance — that’s what keeps the system trustworthy.
  • A single debit or credit is called an operation.
Keep this mental model close. Everything that follows — accounts, balances, transactions, fees — is just this idea, applied.
See it in LerianSee these ideas in practice: Transactions and the Glossary.