Since cyclists' legs are most efficient over a narrow range of pedaling speeds, or cadence , a variable gear ratio helps a cyclist to maintain an optimum pedalling speed while covering varied terrain. Some, mainly utility, bicycles use hub gears with between 3 and 14 ratios, but most use the generally more efficient dérailleur system, by which the chain is moved between different cogs called chainrings and sprockets in order to select a ratio. A dérailleur system normally has two dérailleurs, or mechs, one at the front to select the chainring and another at the back to select the sprocket. Most bikes have two or three chainrings, and from 5 to 11 sprockets on the back, with the number of theoretical gears calculated by multiplying front by back. In reality, many gears overlap or require the chain to run diagonally, so the number of usable gears is fewer.
The explanation of fluctuations in aggregate economic activity is one of the primary concerns of macroeconomics . The main framework for explaining such fluctuations is Keynesian economics . In the Keynesian view, business cycles reflect the possibility that the economy may reach short-run equilibrium at levels below or above full employment . If the economy is operating with less than full employment, ., with high unemployment , Keynesian theory states that monetary policy and fiscal policy can have a positive role to play in smoothing the fluctuations of the business cycle.