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A hardness-intensity diagram (HID) is essentially a type of color-magnitude diagram oriented to X-rays: hardness, generally short for hardness ratio, is essentially a color index within the X-ray spectral band, and intensity is measured as photons received by an instrument during an observation. HIDs are typically used for variable X-ray sources such as an X-ray binary, the plotted points representing separate measurements each representing a time interval observation of the same source, e.g., a plot of a few hours with each plotted point being count of photons collected over one minute.