segregation of duties
There is a reason the person who counts the votes is not allowed to also be a candidate, and the cashier who takes your money is usually not the same person who later checks the till. If one person can do every step of a task alone and in secret, they can both make a mistake and hide it. Segregation of duties is the simple, powerful idea of splitting a sensitive task among different people so that no single person controls the whole thing from start to finish.
Classically, three kinds of responsibility should be kept in separate hands: authorization (approving a transaction), custody (handling the actual asset, like cash or inventory), and recording (entering it in the books). The point is that fraud or error then requires either a slip past someone else or active collusion. Take a payment cycle: the employee who requests a purchase should not be the one who approves it, who should not be the one who signs the cheque, who should not be the one who records the payment, who should not be the one who reconciles the bank account. If one clerk could do all five, they could invent a fake supplier, pay it, record it, and hide it — alone. Spreading those steps out makes that almost impossible without someone else noticing.
Segregation of duties matters because it is one of the cheapest and most effective internal controls there is, and auditors look for it everywhere. The honest limitations are worth stating plainly. First, it can always be defeated by collusion — if the approver and the recorder team up, the wall comes down. Second, small businesses often cannot afford enough staff to separate duties properly, so they must lean on compensating controls instead (such as the owner personally reviewing the bank statement). Segregation reduces risk; it does not erase it.
In a well-run office, the clerk who enters new suppliers cannot also approve invoices or sign cheques; a manager approves, a different person pays, and a fourth reconciles the bank. Because no one controls the whole chain, a single dishonest employee cannot create a fake vendor and quietly pay themselves without someone else having to be in on it.
Splitting authorization, custody, and recording stops one person acting alone.
Segregation of duties is defeated by collusion, and small businesses often can't afford it — they must rely on compensating controls like the owner personally reviewing bank statements.