join syntax

One of my colleagues has created a database where he has the same field name in two tables and uses this field to link his tables rather than some arbitrary value. For example, he has used "exp_id" in two tables. When writing his joins he uses a syntax that says something like JOIN ON EXP_ID. Can someone tell me what that syntax should be? I am not very familiar with it since I typically use the syntax where one field is set equal to the other.
Personally I prefer not to set databases up this way but cannot seem to convince him of this. And yet I am supposed to now help him with his database and application.. Is there some documentation that would define this type of syntax? How is this handled if you have more than one table in the join? It does not appear that this format would allow for this.
Thanks
Jodi