A subsidiary ledger is a group of similar accounts whose combined balances equal the balance in a specific general ledger account. The general ledger account that summarizes a subsidiary ledger's account balances is called a control account or master account. For example, an accounts receivable subsidiary ledger (customers' subsidiary ledger) includes a separate account for each customer who makes credit purchases. The combined balance of every account in this subsidiary ledger equals the balance of accounts receivable in the general ledger. Posting a debit or credit to a subsidiary ledger account and also to a general ledger control account does not violate the rule that total debit and credit entries must balance because subsidiary ledger accounts are not part of the general ledger; they are supplemental accounts that provide the detail to support the balance in a control account.
The accounts receivable subsidiary ledger is essential to most businesses. Companies may have hundreds or even thousands of customers who purchase items on credit, who make one or more payments for those items, and who sometimes return items or purchase additional items before they finish paying for prior purchases. Recording all credit purchases, returns, and subsequent payments in a single account would make an individual customer's balance virtually impossible to calculate because the customer's transactions would be interspersed among thousands of other transactions. But the accounts receivable subsidiary ledger provides quick access to each customer's balance and account activity.
Companies create subsidiary ledgers whenever they need to monitor the individual components of a controlling general ledger account. In addition to the accounts receivable subsidiary ledger, companies often use an accounts payable subsidiary ledger (creditors' subsidiary ledger), which has separate accounts for each creditor, an inventory subsidiary ledger, which has separate accounts for each product, and a property, plant, and equipment subsidiary ledger, which has separate accounts for each long‐lived asset.