What Do Two Symbols in a Square Indicate on a Cross Stitch Pattern?

The stitches should be oriented from the corner of the square to the center, as shown in the image above.

Now that the mystery of the two symbols has been solved, what does the black line across the square indicate? That line may represent stitching a half stitch or a back stitch. The pattern key should give some idea of the correct stitch. The color of the stitch should also be indicated in the key. (To see the two-symbol square in the context of a larger pattern, proceed to the next step.)