We present a statistical investigation of shear motion of the ultraviolet ( UV ) or extreme-ultraviolet (EUV ) footpoints in two-ribbon flares, using the high spatial resolution data obtained in 1998Y2005 by TRACE. To do this study, we have selected 50 well-observed X and M class two-ribbon flares as our sample. All 50 of these flares are classified into three types based on the motions of the footpoints with respect to the magnetic field (SOHO MDI ). The relation between our classification

more »

... me and the traditional classification scheme (i.e., "ejective" and "confined" flares) is discussed. We have found that 86% (43 out of 50) of these flares show both strong-to-weak shear change of footpoints and ribbon separation (type I flares), and 14% of the flares show no measurable shear change of conjugate footpoints, including two flares with very small ribbon separation (type II flares) and five flares having no ribbon separation at all through the entire flare process (type III flares). Shear motion of footpoints is thus a common feature in two-ribbon flares. A detailed analysis of the type I flares shows (1) for a subset of 20 flares, the initial and final shear angles of the footpoints are mainly in the range 50 Y80 and 15 Y55 , respectively; and (2) in 10 of the 14 flares having both measured shear angle and corresponding hard X-ray observations, the cessation of shear change is 0Y2 minutes earlier than the end of the impulsive phase, which may suggest that the change from impulsive to gradual phase is related to magnetic shear change.