Introductory article in a series by Ralph Griswold on t-sequences, the threading and treadling integer sequences found in weaving drafts. It defines their characteristics (positive integers, limited length, small maximum value) and sets out a notation for describing and constructing them, covering terms, variables, sequence names, assignment, ellipses, and the empty sequence. It also introduces grid-plot graphic representations, t-sets and t-numbers, and sequence metrics for length, minimum value and bound. The last page shows grid plots of several t-sequences.