In addition to the classic five types, there are a number of additional types of solar radio bursts. These include variations of the standard types, fine structure within another type, and entirely distinct phenomena. Variant examples include Types J and U bursts, which are Type III bursts for which the frequency drift reverses to go from lower to higher frequencies, suggesting that an electron beam first traveled away and then back toward the Sun along a closed magnetic field trajectory. Fine structure bursts include zebra patterns and fibre bursts that may be observed within Type IV bursts, along with the herringbone bursts that sometimes accompany Type IIs. Type S bursts, which last only milliseconds, are an example of a distinct class. There are also a variety of high-frequency microwave burst types, such as microwave Type IV bursts, impulsive bursts, postbursts, and spike bursts.