Double entry

Double entry has a reputation for being arbitrary. It is not. Every rule in it is forced by the equation in chapter 2, and once that is seen there is nothing left to memorise.

Where the rule comes from

Rearrange the equation so everything is on one side:

$\text{Assets} = \text{Liabilities} + \text{Equity}$

The left side and the right side must move together. So:

• An asset increase must be matched by either another asset decreasing or a liability or equity increasing.
• There is no other possibility, because the identity has to survive.

Debit means the left side. Credit means the right side. That is all the words mean — they are Latin bookkeeping terms, not value judgements. A credit is not good and a debit is not bad.

The rules, derived rather than listed

Account type	Which side of the equation	Increases with
Assets	Left	Debit
Liabilities	Right	Credit
Equity	Right	Credit
Revenue	Right (increases equity)	Credit
Expenses	Right, negatively (reduces equity)	Debit

The last two rows are the ones people memorise and the ones that need no memorising. Revenue increases retained earnings, which is equity, which is on the right — so revenue is credited. Expenses reduce equity, so they go the other way.

Every rule in that table is "which side of the equation does this sit on".

Working the problem

Bought stock for ₹50,000 on credit. An asset arrived; an obligation was created.

	Debit	Credit
Inventory	50,000
Trade payables		50,000

Paid ₹30,000 of it. Cash left; the obligation shrank.

	Debit	Credit
Trade payables	30,000
Cash		30,000

Sold half the stock for ₹40,000 cash. This is two transactions, and seeing that is the whole exercise. Money came in:

	Debit	Credit
Cash	40,000
Revenue		40,000

And stock left:

	Debit	Credit
Cost of goods sold	25,000
Inventory		25,000

Recording only the first half would report ₹40,000 of profit on a sale that actually earned ₹15,000. That second entry is where cost of goods sold comes from, and it is why gross margin exists as a concept at all.

Totals: debits 50,000 + 30,000 + 40,000 + 25,000 = ₹1,45,000. Credits the same.

The trial balance, and its limits

Because every entry has equal debits and credits, the sum of all debits must equal the sum of all credits. Listing every account balance and checking that is a trial balance.

It catches arithmetic and one-sided entries. It does not catch:

• A transaction omitted entirely
• A transaction recorded twice
• The right amount posted to the wrong account
• A deliberate pair of offsetting fictional entries

A balanced trial balance proves the books are internally consistent, not that they are right. Chapter 15's earnings management techniques are almost all perfectly balanced.

Why the system survived six centuries

Three properties, and they are why it is still used when a database could record anything in any shape:

It self-checks. A single mistyped entry breaks the balance and is found.

It records causes, not just amounts. "Cash fell ₹30,000" is a fact. "Cash fell ₹30,000 because a payable was settled" is information, and the second entry is what carries it.

It produces both statements from one record. The balance sheet and the income statement are two views of the same ledger, which is why they cannot disagree.

The point

Debit means left and credit means right, and every rule about which accounts increase on which side falls directly out of assets equalling liabilities plus equity. A sale is two entries — revenue in, inventory out — and recording only the first overstates profit by the entire cost. A trial balance proves the books are internally consistent and proves nothing about whether they are true.