The story I was told and what I believe is that the journal entry is and always is the source of truth. A merchant may have several journals. A separate one for each line of business and maintained by separate clerks. The different journals would then be consolidated into a single ledger. So he can tell what his equity is. When transferring the journal entry to A=L+E. Those early accountants used their version of Excel. For Assets, They took a page and drew a vertical line. For Liabilities, they also drew a vertical line. Same for equity. They called the left side debit and the right side credit. We don’t know why the Italians named it this way. We can only assume the first ledgers dealt with paying down amounts of credit they owed others. Anyways this early “excel” allowed simple ledgers to have two columns. Positive asset changes go to the left and negative to the right. Positive liabilities changes to the right and negative changes to the left. Same thing for equity. I assume this was mantra they told themselves to ensure correctness or reconciliation. When transferring a journal entry to the ledger there must be a debit and a credit or there is fraud. For example an unscrupulous clerk may have taken a loan out. The journal entry may not tell where that money went. When transferring to the ledger, the loan would be entered as a credit. Because the there was not a corresponding debit, either an increase in cash assets or decrease in equity. The balance would have been off and would have told the merchant something was wrong.
