Solar radio bursts are classified largely based on how they appear in dynamic spectrum observations from radiospectrographs. The first three types, shown in the image on the right, were defined by Paul Wild and Lindsay McCready in 1950 using the earliest radiospectrograph observations of metric (low-frequency) bursts. This classification scheme is based primarily on how a burst's frequency drifts over time. Types IV and V were added within a few years of the initial three, and a number of other types and sub-types have since been identified.