Thanks. That's exactly what I needed. It wasn't intuitive that this was necessary, but it appears to be doing what I need.
I agree with @adam. That's called a many-to-many relationship, but since Quickbase only let's us define a one-to-many relationship, you accomplish this by creating two one-to-many relationships using a third table in the "middle." Often that table is called a join or pivot table. Then, on the Instructors table, you can have an embedded report (via a Report Link field) that lists the courses they are eligible to teach.
Oftentimes, you may want to store start and end dates on the pivot table as well, since the Instructor may not teach the given courses indefinitely. That way you can track history over time.
