To describe the concept of transitive trust in a simplified way, assume that agent A trusts agent B, and that agent B trusts agent C, then by transitivity, agent A trusts agent C. Trust transitivity manifests itself in various forms during real life human interaction, but can be challenging to concisely model in a formal way. In this paper we describe principles for expressing and analysing transitive trust networks, and define requirements for their validity. This framework can be used for modelling transitive trust in computerised interactions, and can be combined with algebras and algorithms for computing propagation of both trust and distrust. This is illustrated by an example where transitive trust is mathematically analysed with belief calculus.