What it is
The stochastic oscillator, popularised by George Lane from the late 1950s onward, asks a single question: where does the current close sit inside the high-low range of the last N bars? The premise is that closes cluster near the top of the range while buyers are in control and near the bottom when sellers are.
How it works
Raw %K is 100 x (close - lowest low over N) / (highest high over N - lowest low over N), typically with N = 14. %D is a three-period average of %K and serves as the signal line. The slow stochastic that most platforms display by default smooths %K over three periods first and then takes %D of that result, which removes much of the jitter of the raw version without changing what it measures.
How traders use it
Readings above 80 and below 20 mark the upper and lower parts of the recent range. In balanced, range-bound markets the classic use is to fade those extremes on a %K/%D crossover, ideally at a level that already matters for other reasons. Divergence is a secondary read, and more useful still is the failure of the oscillator to reach the opposite extreme on a pullback, which suggests the range is losing symmetry.
Where it breaks down
In a trend the oscillator becomes embedded: it pins near 100 or near 0 for long stretches because every close prints near the edge of the window, and every countertrend crossover is a losing trade. Fast settings on low timeframes generate signals almost continuously. Treat it as a range tool that has to be switched off — by ADX, by a moving-average filter, by anything reliable — once a trend establishes itself.
Educational reference. This entry describes how a concept is defined and used. It is not investment advice, not a recommendation, and not a signal. Any rule you build from it should be tested with realistic costs before it is traded, and no historical result guarantees a future one.