The ligule is an extension of the outer layer of the sheath beyond the junction of the leaf petiole. The length and shape of the ligules are very important characters used to distinguish species of Costus. The ligules of some species are very short, others are quite long and this feature tends to be quite consistent within different populations of a given species. In the identifier tool we include the ligule length as one of the characters for identification. In the full written descriptions of species, we also indicate the shape of the ligules. Some species have long lobes on one or both sides of the sheath, sometimes triangular in shape, sometimes rounded. Some species do not have lobes but are of equal lenth all around the sheath - said to be "truncate".
Below are some example photos of ligules, showing how to measure the length.