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If you’ve ever looked at a banking system and felt lost the moment someone said “debit” and “credit”, start here. You don’t need an accounting background. You need one idea, and once it clicks, almost everything else in core banking falls into place. Money never appears out of nowhere, and it never 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 appears, nothing disappears. 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: the system records every movement on both sides, once as it leaves and once as it arrives. A single-sided record makes mismatches harder to detect. Recording both sides gives the system an arithmetic constraint to check.

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The two sides have names: debit and credit

The two sides of a movement have traditional names. In this movement, they describe the recorded source and destination:
  • 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.
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. In accounting more broadly, debit and credit are sides of an entry. Whether either increases or decreases a balance depends on the account type and its configured direction. 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. For the transfer shown here, debit is the source and credit is the destination. Don’t treat that example as a universal rule for every account type.

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Why both sides are always required

Because the system records every movement 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. The system won’t let the movement through. This rule (the two sides must balance) is a consistency control. Midaz validates that the source total, the destination total, and the sent value all match. This check catches a recorded amount mismatch, but it does not by itself prove authorization, classification, or completeness. You can still trace every recorded movement to its source and destination.
Each individual part of a movement, a single debit or a single credit, is an operation. A movement of money consists of operations that balance out.

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Next steps

See it in LerianSee these ideas in practice: Transactions and the Glossary.