Macro theory
UK economist Lord Keynes developed a theory in which prices failed to adjust quickly so that a fall in corporate investment or a rise in savings leads to a decline in output.
Hicks developed IS-LM model, a mathematical version of Keynes thinking which is still the baseline framework for thinking about business cycles.
For 20-30 years, most macro was about measuring the gap between demand and potential output and stimulating demand sufficiently to reach potential.