The term ''planar magnetic structure'' (PMS) refers to distinctive features in the solar wind characterized by a series of abrupt changes in the direction of the interplanetary field, with the field remaining nearly parallel to a single plane. Previous suggestions concerning the origin of PMS are (1) closed magnetic field loops arising from newly emerging, nearly parallel magnetic structures in the photosphere, (2) compression and draping of the field around a high-speed coronal mass ejection (CME), and (3) extended current sheets and/or loops from multiple helmet streamers in the corona. Four years of ISEE 3 data have been examined to establish a larger data base of PMS events and to allow a search for common features that may reveal the causes of these structures. A total of 33 PMS events were found and analyzed. The loop theory of PMS origins was tested by searching for a correlation between the PMSs and the electron or proton counterstreaming characteristic of closed loops of CMEs. It was found that the PMSs tended to precede rather than coincide with the CMEs. Fourteen of the PMS events (42%) were in the sheath between an interplanetary shock and a CME. Other PMS events were found to be related to crossings of the heliospheric current sheet (HCS). Sorting the events according to their association with either HCS or compression/draping ahead of fast streams (of which postshock sheaths are a subset) yields 19 events (58%) associated with compression/draping but not with HCS, 8 events (24%) associated with both compression/draping and HCS, 5 events (15%) associated with HCS but not with compression/draping, and 1 event (3%) associated with neither compression/draping nor the HCS. The conclusion is that planar magnetic structures are not closed loops of plasma from coronal mass ejections or helmet streamers. Instead, a process is postulated in which coronal mass ejections lead to the amplification and alignment of preexisting discontinuities in the ambient wind compressed by the CMEs. A possible cause of the alignment of discontinuities in the vicinity of the heliospheric current sheet is compression by lateral expansion of CMEs in the lower corona. ¿ American Geophysical Union 1993