Periodically you pause, pulling back a bit to inspect your work, setting down the pencil, taking up the eraser, editing your grid of hashmarks, erasing, making a new mark just to the left of the old, or below it, aiming not for machinic uniformity but for a quality of facture that evokes the care you wish you brought to all things. When you are satisfied, when the sheet of paper is covered in a grid of marks that extends uniformly from edge to edge in either dimension, the erasures lending the thing a pathos, like a face softened by age, set down your pencil and eraser and again take the sheet in your hands. This time fashion a tube in the other dimension — if you’ve had the sheet in a portrait orientation, so that your initial series of marks ran down the longer dimension and when you joined the edges it was the shorter edges you were joining, now you will be joining the longer edges. Or perhaps your sheet is square — in any case, whichever dimension you manipulated in the earlier exercises, this time start with the other. When you have satisfied yourself, by tracing a path from mark to mark with your eyes or the index finger of your free hand, that you have induced periodicity in this dimension, unroll the sheet of paper and try it in the other. Note how it changes things to have a grid of marks rather than a series.