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This paper explores the connection between semantic equivalences and preorders for concrete sequential processes, represented by means of labeled transition systems, and formats of transition system specifications using Plotkin's structural approach. For several preorders in the linear timebranching time spectrum a format is given, as general as possible, such that this preorder is a precongruence for all operators specifiable in that format. The formats are derived using the modal characterizations of the corresponding preorders.

The importance of giving precise semantics to programming and specification<br />languages was recognized since the sixties with the development of the<br />first high-level programming languages (cf. e.g. [30, 206] for some early accounts).<br />The use of operational semantics - i.e. of a semantics that explicitly<br />describes how programs compute in stepwise fashion, and the possible<br />state-transformations they perform - was already advocated by McCarthy<br />in [147], and elaborated upon in references like [142, 143]. Examples of full-blown<br />languages that have been endowed with an operational semantics are<br />Algol 60 [140], PL/I [173], and CSP [178].

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