The phrase "proper to" means suitable or appropriate for a particular situation or circumstance. There are several synonyms that can be used in place of this phrase, such as "fitting," "relevant," "pertinent," "applicable," "adequate," and "suitable." These synonyms convey a sense of appropriateness and suggest that something is well-suited to a specific situation. Similarly, other synonyms including "apposite," "germane," and "customary" carry the connotation that something is not only proper, but also expected or customary in a given context. Using these synonyms can help add variety to your language and convey nuance in your writing or speaking.