Double-entry bookkeeping: debits, credits, the journal and the ledger

Lesson 2 analysed transactions directly in the accounting equation, with pluses and minuses under each element. That works for a dozen transactions, but a real business records hundreds or thousands each month, and it needs a system that keeps every transaction traceable, keeps the books in balance automatically, and produces totals for each item. Double-entry bookkeeping, developed by Italian merchants and described by Luca Pacioli in 1494, is that system. It records every transaction with at least one debit and at least one credit of equal total, first in a chronological journal and then in a ledger organized by account. This lesson explains accounts, the debit and credit rules, journal entries, posting to the ledger and the chart of accounts, using the same May transactions for Birchwood Bikes.

Accounts and the T-account

An account is a record of the increases and decreases in one item: cash, accounts receivable, equipment, accounts payable, service revenue, rent expense and so on. The simplest picture of an account is a T-account, drawn as a large letter T with the account name on top. The left side is called the debit side and the right side the credit side. Debit and credit simply mean left and right. They do not mean good or bad, increase or decrease, or anything to do with a bank's use of the words on your bank statement. Whether a debit increases or decreases an account depends on the kind of account.

The rules of debit and credit

The rules follow from the accounting equation. Assets are on the left of the equation, so assets increase on the left: a debit increases an asset and a credit decreases it. Liabilities and equity are on the right, so they increase on the right: a credit increases a liability or equity and a debit decreases it. Because revenues increase equity, revenues increase with credits. Because expenses and dividends decrease equity, they increase with debits. Each account's normal balance is the side on which it increases.

Because every transaction is recorded with equal debits and credits, total debits always equal total credits across all accounts, and the accounting equation stays in balance. This gives bookkeeping a built-in check, the trial balance in lesson 4. It also means that once you know one side of an entry, you can usually work out the other. If cash is received, cash is debited, and something must be credited: revenue if it was earned, a receivable if a customer paid an old bill, a liability if the business borrowed or the customer paid in advance, or share capital if an owner invested.

The journal

Transactions are first recorded in a journal, the book of original entry, in date order. Each journal entry shows the date, the accounts debited and credited with their amounts, and a short explanation, often with a reference to the source document. By convention, debits are listed first and credits are indented beneath them. The general journal can record any transaction; many businesses also use special journals or subledgers for frequent transactions such as sales, purchases, cash receipts and payroll, and accounting software keeps these as separate modules that feed the general ledger.

Here are the May journal entries for Birchwood Bikes, using the transactions from lesson 2. Read each entry and say in words why each account is debited or credited. That habit of explaining each line is the fastest way to become fluent.

The 31 May rent and wages entry has two debits and one credit; it is a compound entry. That is fine as long as total debits equal total credits. Note also that no entry ever records 'revenue' when cash is collected on an old receivable (28 May) or 'expense' when an old payable is paid (25 May). Those entries only move amounts between asset and liability accounts.

Posting to the ledger

The ledger is the collection of all the business's accounts. Posting is the process of transferring each journal entry's debits and credits to the accounts they affect. After posting, each account shows its running balance, and the ledger can produce a list of all balances at any date. In a manual system, posting is a separate step with cross-references: the ledger shows the journal page each amount came from, and the journal shows the account number each amount was posted to. In accounting software, posting happens instantly when an entry is saved, but the logic is the same, and the audit trail lets you click from a ledger balance back to the source entry and document.

# A minimal double-entry ledger (illustrative)
from collections import defaultdict

NORMAL = {"Cash": "Dr", "Accounts receivable": "Dr", "Supplies": "Dr",
          "Prepaid insurance": "Dr", "Equipment": "Dr",
          "Accounts payable": "Cr", "Note payable": "Cr",
          "Share capital": "Cr", "Service revenue": "Cr",
          "Rent expense": "Dr", "Wages expense": "Dr",
          "Utilities expense": "Dr", "Dividends": "Dr"}

journal = [
    ("May 1",  [("Cash", 40000)], [("Share capital", 40000)]),
    ("May 2",  [("Cash", 20000)], [("Note payable", 20000)]),
    ("May 3",  [("Equipment", 18000)], [("Cash", 18000)]),
    ("May 5",  [("Prepaid insurance", 3600)], [("Cash", 3600)]),
    ("May 8",  [("Supplies", 1500)], [("Accounts payable", 1500)]),
    ("May 15", [("Cash", 4200)], [("Service revenue", 4200)]),
    ("May 20", [("Accounts receivable", 2800)], [("Service revenue", 2800)]),
    ("May 25", [("Accounts payable", 1000)], [("Cash", 1000)]),
    ("May 28", [("Cash", 1800)], [("Accounts receivable", 1800)]),
    ("May 31", [("Rent expense", 2000), ("Wages expense", 3100)], [("Cash", 5100)]),
    ("May 31", [("Utilities expense", 350)], [("Accounts payable", 350)]),
    ("May 31", [("Dividends", 500)], [("Cash", 500)]),
]

ledger = defaultdict(lambda: [0, 0])          # account -> [debits, credits]
for date, dr, cr in journal:
    assert sum(a for _, a in dr) == sum(a for _, a in cr), date
    for acct, amt in dr: ledger[acct][0] += amt
    for acct, amt in cr: ledger[acct][1] += amt

for acct in NORMAL:
    d, c = ledger[acct]
    bal = d - c if NORMAL[acct] == "Dr" else c - d
    print(f"{acct:<20} Dr {d:>7,}  Cr {c:>7,}  balance {bal:>7,} {NORMAL[acct]}")

Cash                 Dr  66,000  Cr  28,200  balance  37,800 Dr
Accounts receivable  Dr   2,800  Cr   1,800  balance   1,000 Dr
Supplies             Dr   1,500  Cr       0  balance   1,500 Dr
Prepaid insurance    Dr   3,600  Cr       0  balance   3,600 Dr
Equipment            Dr  18,000  Cr       0  balance  18,000 Dr
Accounts payable     Dr   1,000  Cr   1,850  balance     850 Cr
Note payable         Dr       0  Cr  20,000  balance  20,000 Cr
Share capital        Dr       0  Cr  40,000  balance  40,000 Cr
Service revenue      Dr       0  Cr   7,000  balance   7,000 Cr
Rent expense         Dr   2,000  Cr       0  balance   2,000 Dr
Wages expense        Dr   3,100  Cr       0  balance   3,100 Dr
Utilities expense    Dr     350  Cr       0  balance     350 Dr
Dividends            Dr     500  Cr       0  balance     500 Dr

The program refuses any entry whose debits do not equal its credits, posts each line to its account, and prints each account's total debits, total credits and balance on its normal side. Cash, for example, has debits of $66,000 and credits of $28,200, leaving a debit balance of $37,800, the same figure lesson 2 found in the equation. Lesson 4 turns these balances into a trial balance.

The chart of accounts

A chart of accounts is the list of all accounts a business uses, with a number for each. Numbers are usually grouped by type so that the order matches the financial statements: for example, 100s for assets, 200s for liabilities, 300s for equity, 400s for revenues and 500s and above for expenses. Gaps are left so that new accounts can be added. A well-designed chart has enough detail to produce the reports managers and tax authorities need, but not so many accounts that people guess which one to use. Software often comes with a template chart for the type of business, which the owner then adjusts.

Why the system has lasted

Double entry has survived five centuries because it is self-checking, complete and traceable. Every amount appears twice, so many errors show up as an imbalance. Every transaction is recorded in date order in the journal and summarized by account in the ledger, so you can answer both 'what happened on 15 May?' and 'what happened to cash this month?'. And every balance can be traced to entries and every entry to a source document, which is what auditors, tax authorities and managers investigating a problem rely on. Modern databases store transactions differently, but they still enforce the rule that debits equal credits.