I've always gone back and forth on this question. I understand that at least to some degree, the answer is going to be based on context. However, I'd like to know what others think as well. Here's a sentence I'll use as an example: Jan sat at a table in the library surrounded by stacks of old files and books covered with dust that made her nose itch. The clause, "that made her nose itch," refers to what, exactly? I wrote it to refer to the dust. However, when I read it again, I realized that without any other commas, it is at least possible to read this line as both the old files, and dust-covered books making her nose itch. So, when you all read something like this, how do you determine what is being referred to?