We describe and interpret new results obtained by correlating measurements of solar wind speed, density, and flux made by the plasma instruments on the SOHO and Wind spacecraft at separations between ¿150 Earth radii (RE) in the Y (GSE) direction and up to 280 RE in the X (GSE) direction. These measurements cover the period from 1996 to mid-1998 and all of 1999, a total of 3.5 years. We have correlated observations spaced 96 s apart over 2-hour intervals and observations spaced 300 s apart over 6-hour intervals. Typical average correlation coefficients for solar wind flux are 0.6¿0.2 with 10% of the 2-hour intervals and 22% of the 6-hour intervals having correlation coefficients >0.8. Comparing the 2- and 6-hour results, correlation coefficients are systematically smaller for the higher time resolution data. This is a result of a larger random data component and fewer solar wind plasma parameter changes in the 2-hour data compared to the 6-hour data. We interpret the results in terms of inclined planar fronts which provide an attractive and useful way of organizing the data. The front azimuth angles are, on average, in the same quadrant as the average Parker spiral angle. A limited three-dimensional analysis of the fronts shows their normals to be inclined, on average, by a few tens of degrees to the ecliptic plane. Possible factors that could control front angles are examined, and long-term correlations with solar activity are discussed. The effect of discontinuities in the interplanetary medium is also discussed. ¿ 2001 American Geophysical Union