Overview

• What you’ll learn: The principles of double-entry bookkeeping, debit and credit rules for each account type, how to use T-accounts, and why every transaction has two sides.
• Prerequisites: Lesson 2 — The Accounting Equation
• Estimated reading time: 18 minutes

Introduction

The Grand Historian records: In the year 1494, a Franciscan friar named Luca Pacioli published a treatise that would echo through the centuries. Within his mathematical compendium Summa de Arithmetica, he described a system already practiced by Venetian merchants — the method of keeping books in two columns, where every entry on one side demanded a corresponding entry on the other. He called it the Method of Venice. We call it double-entry bookkeeping.

This system is so elegant that it has survived over five hundred years without fundamental change. Empires have risen and fallen, currencies have come and gone, and entire industries have been born and destroyed — yet the basic principle remains: for every debit, there must be a credit. It is the accounting equivalent of Newton’s Third Law, and it is just as unbreakable.

The Concept of Debits and Credits

Let us dispel the most common confusion immediately: in accounting, “debit” does not mean “bad” and “credit” does not mean “good.” These are merely directional terms — left and right, nothing more. A debit is an entry on the left side of an account; a credit is an entry on the right side.

The Rules of Debits and Credits

The mnemonic DEALER may help: Dividends, Expenses, Assets are increased by debits; Liabilities, Equity, Revenue are increased by credits.

T-Accounts: The Visual Tool

A T-account is a simplified representation of a ledger account, shaped like the letter T:

        Account Name
    ─────────┬─────────
     Debit   │  Credit
     (Left)  │  (Right)

Every account in the chart of accounts can be represented as a T-account. The left side records debits; the right side records credits. The balance is the difference between the two sides.

Example: Cash T-Account

            Cash
    ─────────┬─────────
     10,000  │    800
      2,000  │  3,000
    ─────────┼─────────
      8,200  │
    (Balance) │

Debits total $12,000; credits total $3,800. The balance is $8,200 debit — meaning the company has $8,200 in cash.

The Dual Nature of Every Transaction

Every transaction affects at least two accounts. This is not optional — it is the very definition of double-entry bookkeeping. Consider:

Transaction 1: Owner invests $10,000

  Debit:  Cash            $10,000
  Credit: Owner's Capital $10,000

Cash (an asset) increases with a debit. Owner’s Capital (equity) increases with a credit. The equation stays balanced.

Transaction 2: Purchase equipment for $3,000 cash

  Debit:  Equipment  $3,000
  Credit: Cash       $3,000

One asset increases (Equipment) while another decreases (Cash). Total assets remain unchanged.

Transaction 3: Earn $2,000 service revenue on account

  Debit:  Accounts Receivable  $2,000
  Credit: Service Revenue      $2,000

An asset increases (Accounts Receivable) and equity increases through revenue. Both sides of the equation grow equally.

Transaction 4: Pay $800 rent expense

  Debit:  Rent Expense  $800
  Credit: Cash          $800

An expense increases (reducing equity) and cash decreases. Both sides shrink equally.

Compound Entries

Some transactions affect more than two accounts. These are called compound journal entries. For example, purchasing a $5,000 vehicle with $2,000 cash and $3,000 on a note payable:

  Debit:  Vehicle        $5,000
  Credit: Cash           $2,000
  Credit: Notes Payable  $3,000

The golden rule still holds: total debits ($5,000) must equal total credits ($2,000 + $3,000 = $5,000).

Common Mistakes to Avoid

• Confusing debit with “decrease”: Debits increase assets and expenses, but decrease liabilities, equity, and revenue.
• Recording only one side: Every transaction requires at least two entries. One-sided entries destroy the balance.
• Mixing up normal balances: An asset account with a credit balance (unless it is a contra account) signals an error.
• Forgetting compound entries: Not all transactions involve exactly two accounts.

Key Takeaways

• Double-entry bookkeeping requires every transaction to be recorded with equal debits and credits.
• Debit means left; credit means right. Neither is inherently good or bad.
• Assets, expenses, and dividends have normal debit balances. Liabilities, equity, and revenue have normal credit balances.
• T-accounts provide a visual way to track debits and credits for each account.
• Compound entries involve more than two accounts but total debits must still equal total credits.

What’s Next

In Lesson 4, you will learn about the Chart of Accounts — the organized listing of every account a business uses, complete with numbering systems and account classifications.

        帳戶名稱
    ─────────┬─────────
     借方    │  貸方
     （左）  │  （右）

  借：現金          $10,000
  貸：業主資本      $10,000

  借：設備    $3,000
  貸：現金    $3,000

  借：車輛          $5,000
  貸：現金          $2,000
  貸：應付票據      $3,000

        勘定科目名
    ─────────┬─────────
     借方    │  貸方
     （左）  │  （右）

  借方：現金        $10,000
  貸方：資本金      $10,000

  借方：設備    $3,000
  貸方：現金    $3,000

  借方：車両        $5,000
  貸方：現金        $2,000
  貸方：支払手形    $3,000