otter is based on resolution and paramodulation, constrained by term orderings similar to those in the superposition calculus. the prover also supports positive and negative hyperresolution and a set-of-support strategy. proof search is based on saturation using a version of the given-clause algorithm, and is controlled by several heuristics. there also are meta-heuristics determining search parameters automatically. otter also pioneered the use of efficient term indexing techniques to speed up the search for inference partners in large clause sets.