At one extreme, we may assume that distances in only the column direction and not in the row direction affect the covariance between residuals. To show what this R structure looks like, assume we have 100 observations measured on a grid of 10 rows × 10 columns. If we sort the observations by row position and column within row and fit an AR1 correlation (ρ) only in the row direction, we have an R structure as described in Figure 3-1: