The name "trellis" derives from the fact that a state diagram of the technique closely resembles a trellis lattice. The scheme is basically a convolutional code of rates ("r", "r"+1). Ungerboeck's unique contribution is to apply the parity check for each symbol, instead of the older technique of applying it to the bit stream then modulating the bits. He called the key idea "mapping by set partitions". This idea groups symbols in a tree-like structure, then separates them into two limbs of equal size. At each "limb" of the tree, the symbols are further apart.