Keynesian cross

Here is a deceptively simple idea: an economy produces exactly as much as people plan to buy. If everyone together plans to spend more than is currently being produced, businesses see their shelves empty out, so they ramp up production; if planned spending is less than what's produced, inventories pile up unsold and firms cut back. The Keynesian cross is a diagram that finds the one level of output at which planned spending exactly matches production — the economy's short-run resting point.

The diagram plots total planned spending against total income (output) and draws two lines. One is a 45-degree line, where spending would equal income exactly. The other is the planned-spending line — consumption plus investment plus government plus net exports — which rises with income but less than one-for-one, because people save part of any extra income. Where these two lines cross is equilibrium output: the point where what people plan to spend equals what the economy produces. A core assumption is that, in this short-run picture, prices are fixed, so all the adjustment happens through quantities (output and jobs) rather than prices. That assumption is exactly what makes it a short-run, recession-focused tool.

The Keynesian cross is the simplest home of two big ideas: that an economy can get stuck below full employment if planned spending is too weak, and that a change in spending can have an amplified effect on output — the multiplier. It was the backbone of mid-20th-century demand-management thinking. Its honest limitation is that fixed prices and a single interest rate are heroic simplifications; richer models (like IS-LM and AD-AS) add the missing pieces. But as a first lens on why weak demand causes recessions, it is hard to beat.