Across (The Set)

In Scottish Country Dancing Across the set means at right angles to the Side lines of a Longwise set or to the Centre line of a Square set.

For example, here are bars 9 to 16 of The Black Leather Jig -
1s Dance Left shoulder reel of three Across the set (Man with 2s and Lady with 3s) and 1s Turn By the left to Face First corners.