This section is from the book "Elementary Banking", by John Franklin Ebersole. Also available from Amazon: Elementary Banking.
A control account is an account which controls others called auxiliary or subsidiary accounts. The important feature of this control is that the controlling account varies in balance as does the sum of the balances of the auxiliary or supporting accounts; that is, figures affecting the control affect the auxiliary accounts similarly. A depositors' account on the general ledger controls the individual depositors' accounts on the depositors' ledger. At the close of each day the total credit balances shown by the depositors' ledger or ledgers, less the overdrafts or debit balances, equals the balance of the depositors' control account on the general ledger. Practically all general ledger accounts are control accounts.