Trust Relationship
A trusted and logical connection between two ADs or domain to share the resources of each other domain,, called trust relationship It has 2 types .One Way relationship .Two way relationship
Multimaster domain trust
two-way transitive
two-way transitive for domain in the same forest
because of sex
Explicit one-way trust
external trust Use external trusts to provide access to resources located on a Windows NT 4.0 domain or a domain located in a separate forest that is not joined by a forest trust When a trust is established between a domain in a particular forest and a domain outside of that forest, security principals from the external domain can access resources in the internal domain. Active Directory creates a foreign security principal object in the internal domain to represent each security principal from the trusted external domain. These foreign security principals can become members of domain local groups in the internal domain. Domain local groups can have members from domains outside of the forest. Directory objects for foreign security principals are created by Active Directory and should not be manually modified. You can view foreign security principal objects from Active Directory Users and Computers by enabling advanced features. For information about enabling advanced features, see To view advanced features. In domains with the functional level set to Windows 2000 mixed, it is recommended that you delete external trusts from a domain controller running Windows Server 2003. External trusts to Windows NT 4.0 or 3.51 domains can be deleted by authorized administrators on the domain controllers running Windows NT 4.0 or 3.51. However, only the trusted side of the relationship can be deleted on the domain controllers running Windows NT 4.0 or 3.51. The trusting side of the relationship (created in the Windows Server 2003 domain) is not deleted, and although it will not be operational, the trust will continue to display in Active Directory Domains and Trusts. To remove the trust completely, you will need to delete the trust from a domain controller running Windows Server 2003 in the trusting domain. If an external trust is inadvertently deleted from a domain controller running Windows NT 4.0 or 3.51, you will need to recreate the trust from any domain controller running Windows Server 2003 in the trusting domain
"the trust relationship between this workstation and the primary domain failed," ...
Use external trusts to provide access to resources located on a Windows NT 4.0 domain or a domain located in a separate forest that is not joined by a forest trust When a trust is established between a domain in a particular forest and a domain outside of that forest, security principals from the external domain can access resources in the internal domain. Active Directory creates a foreign security principal object in the internal domain to represent each security principal from the trusted external domain. These foreign security principals can become members of domain local groups in the internal domain. Domain local groups can have members from domains outside of the forest. Directory objects for foreign security principals are created by Active Directory and should not be manually modified. You can view foreign security principal objects from Active Directory Users and Computers by enabling advanced features. For information about enabling advanced features, see To view advanced features. In domains with the functional level set to Windows 2000 mixed, it is recommended that you delete external trusts from a domain controller running Windows Server 2003. External trusts to Windows NT 4.0 or 3.51 domains can be deleted by authorized administrators on the domain controllers running Windows NT 4.0 or 3.51. However, only the trusted side of the relationship can be deleted on the domain controllers running Windows NT 4.0 or 3.51. The trusting side of the relationship (created in the Windows Server 2003 domain) is not deleted, and although it will not be operational, the trust will continue to display in Active Directory Domains and Trusts. To remove the trust completely, you will need to delete the trust from a domain controller running Windows Server 2003 in the trusting domain. If an external trust is inadvertently deleted from a domain controller running Windows NT 4.0 or 3.51, you will need to recreate the trust from any domain controller running Windows Server 2003 in the trusting domain
If you're in a relationship, then yes you should be able to trust them. If you can't trust them, your relationship is unhealthy and you shouldn't be in it.
it is when the domain is a whole number