  This paper explores the connection between semantic equivalences and
preorders for concrete sequential processes, represented by means of labelled
transition systems, and formats of transition system specifications using
Plotkin's structural approach. For several preorders in the linear time -
branching 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.
