You can define these sides of the trust in the reverse order, but the order of trusted first and trusting second allows the trust to take effect and be verified immediately. Microsoft has defined four models of domain and trust usage. These models help us talk about different configuration approaches. The simplest and first model is the Single domain model. As its name implies, this model has only one domain and hence no trusts.
All accounts reside in that one domain.
The second model is the Master domain model, which consists of one master domain containing all user accounts and one or more resource-only domains. Each resource domain has no user account, and trusts the master domain. Typically, the resource domains contain file servers, printers, databases, etc. A user logs on to his account on the master domain, and because of all the resource trusts, can have access to all enterprise resources. The third model, the Multiple Master domain model, is a variation on the second.