The phrase "is proper" can be expressed in numerous ways, depending on the context. Some common synonyms that convey a similar meaning include "is correct," "is suitable," "is appropriate," "is fitting," and "is right." These terms indicate that something is done in accordance with established norms, rules, or standards, or in a manner that conforms to expectations. Other synonyms for "is proper" might depend on specific situations or subject matter; for example, in a legal context one might use "is lawful" or "is valid," while in a social context one might use "is polite" or "is respectful." Ultimately, the choice of synonym depends on the speaker's intent and the desired nuance of the message.