Was inflation low or high?
In 1949 Bill Phillips, a New Zealand engineer studying at the London School of Economics, built an economy out of pipes and pink water, a machine for teaching. In 1958 he plotted British wage rises against unemployment for 1861 to 1913: when workers were scarce, wages jumped; when jobs were scarce, they barely moved. He called his conclusions tentative.
In 1960 Paul Samuelson and Robert Solow drew an American version and captioned it a "menu of choice": a little more inflation for a little less unemployment. They also warned that the menu might not keep its shape.
In December 1967 Milton Friedman told the American Economic Association: "there is always a temporary trade-off between inflation and unemployment; there is no permanent trade-off". It works only while inflation surprises people. Once they expect it, they ask for it in their wages, and the curve moves. Edmund Phelps reached the same idea on his own.
In 1975 US unemployment was 8.5% and inflation 9.1%, both at once. By 1980 inflation was 13.5% with unemployment at 7.1%. Oil shocks pushed prices up too, but the old curve no longer described the data.
A pattern in planets stays put when you learn it. A pattern in people can move, because they learned it.
The tanks are our drawing, not the real machine. The 1861-1913 dots are a sketch around Phillips's fitted curve, and the old US curve is a sketch through the 1960s points; the other US dots are annual averages from the Bureau of Labor Statistics.
Did you guess low? What pattern around you changed once everyone knew it?
Sources: A. W. Phillips, "The Relation between Unemployment and the Rate of Change of Money Wage Rates in the United Kingdom, 1861-1957", Economica 25(100), 1958; Paul Samuelson and Robert Solow, "Analytical Aspects of Anti-Inflation Policy", American Economic Review 50(2), 1960; Milton Friedman, "The Role of Monetary Policy", American Economic Review 58(1), 1968; US Bureau of Labor Statistics, unemployment rate and consumer price index.
Sounds: our own, made in code (earshot, CC0).