Swing High / Swing Low

A swing high is a bar whose high exceeds the highs of the bars around it. A swing low is the mirror. The simplest formal definition uses a symmetric width w:

swing high at bar i ⇔ high(i) > high(i±1 … i±w)
swing low  at bar i ⇔ low(i)  < low(i±1 … i±w)

(w = 2 gives the classic 5-bar fractal. Larger widths find fewer, more significant swings.)

The confirmation lag

By definition, a swing needs w bars after the extreme before it can be confirmed. You never know a swing formed until later. Honest backtests must respect this; treating a swing as known at its own bar is look-ahead bias, and one of the subtler ways backtests cheat.

What swings are for

Confirmed swings are the reference points of market structure analysis. Sequences of higher highs and higher lows define trend. Clusters of swings mark support and resistance. The break of a significant swing is a break of structure. And stops naturally live just beyond the swing that justified the trade.

On AlphaProve

williams_fractals in the indicator library is the symmetric-width swing detector the definition above describes, ready on ctx.indicators without hand-rolling the neighbor comparison. The confirmation lag is enforced for you rather than left to discipline: the engine decides on bar close and fills on the next open, and higher-timeframe series aggregate with no lookahead, so a swing cannot leak into a decision before the bars that confirm it have printed.