  We address the question of characterising the well-formedness properties of
multiparty session types semantically, i.e., as properties of the semantic
model used to interpret types. Choosing Prime Event Structures (PESs) as our
semantic model, we present semantic counterparts for the two properties that
underpin global type well-formedness, namely projectability and boundedness, in
this model. As a first step towards a characterisation of the class of PESs
corresponding to well-formed global types, we identify some simple structural
properties satisfied by such PESs.
