How do you add up everything a country makes, and why did a war make it urgent?

You add up everything a country makes by counting it in a way that cannot fool itself. Statisticians build national accounts: a set of books, much like a company's, but for a whole nation. The books use double-entry accounting, which makes the totals on both sides equal even though they measure different things, such as production and the income that production creates. The best-known total is gross domestic product, GDP.

There are three roads to the same number. You can add up what every business produced, you can add up everyone's incomes, or you can add up everything that was bought. In theory they should agree, because every product is bought by somebody and every sale becomes someone's income. The spending road splits into four parts: consumption, investment, government spending and net exports (exports minus imports).

One rule keeps the count honest: only final goods count. If a carmaker buys parts, assembles a car and sells it, only the finished car is added, not the parts too. Otherwise the same steel would be counted again and again.

Around 1930 these books were only just being invented. The Great Depression made accurate measures of the whole economy urgent, and an economist named Simon Kuznets did national income work in the early 1930s so thorough that it set the standard in the field. Then came the war. Kuznets spent 1942 to 1944 at the War Production Board, using national income accounting to ask how much more weaponry the country could make and which materials would run short first. That wartime role helped GDP win its place as the number a nation checks itself against.