A trellis is a graph of which the nodes are ordered into vertical slices (time) and each node at each time is connected to (at least) one node at an earlier and (at least) one node at a later time. The earliest and latest times in the trellis have only one node. Trellises are used in encoders and decoders for communication theory and encryption. They are also the central datatype used in Baum-Welch algorithm for Hidden Markov Models.