Open, High, Low and Close (OHLC)

Every bar on a price chart records four numbers for the period it covers: the first price of the period (the open), the highest and lowest prices reached during it (the high and the low), and the last price before the period ended (the close). Those four values are the entire content of the bar. Everything drawn on top of them — candles, averages, oscillators, named patterns — is computed from them, so a study is only ever as informative as the four numbers underneath it. Two consequences are easy to miss. The four values compress a whole period into a summary and discard the order of what happened inside it: the same open, high, low and close can be produced by a quiet drift or by a violent round trip, and the chart cannot tell them apart without dropping to a shorter period. And in spot forex those four values belong to one feed rather than to a market. There is no central record of transactions, so the high your platform stored for an hour need not be the high another platform stored for the same hour on the same pair. Which prices a provider builds its bars from is stated in that provider's own documentation.