There are a plethora of synonyms for the phrase "line of joining" which can be used interchangeably in various contexts. Some of these synonyms include: connecting line, convergence line, meeting point, intersection, junction, boundary line, seam, merging point, confluence, and interface. All these synonyms convey the same idea of two or more things coming together to form one single entity. Whether it's in the context of engineering, mathematics, or even art, these synonyms make it easier to express ideas clearly and succinctly. So, whether you're writing an essay, a technical report, or just having a conversation, knowing these synonyms will help you to communicate more effectively.