A 2-way sequential transducer is a 2-way finite acceptor to which an output structure has been added. Such a device is considerably more complex than the usual 1-way sequential transducer, and it is shown that these transducers map both regular sets and context free languages into the context sensitive languages. Such a transducer is the simplest known which can map a list into its reversal or make duplicate copies of a list. In this paper unsolvability of equivalence is shown for a special class of nondeterministic 2-way sequential transducers, and it is shown that deterministic transducers are not so powerful as nondeterministic transducers. The properties of two-way transductions of regular-sets and context free languages are derived in detail and are shown to be similar to the corresponding properties of context free languages. Also, in this paper the intimate relationship between 2-way sequential transducers and 1-way stack automata is shown by presenting a new grammar which generates exactly the 1-way stack automata languages, and many of the properties of these grammars are derived.
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