My students always struggled with this. The diagrams are hard to understand. It was difficult to tell whether they were struggling with naming the line or understanding the concept. I wasn't very good at explaining it, I guess. I held a big box, I waved my arms around... they didn't get it.
We made a little box out of paper and glued it into the notebooks. Then we smooshed it flat so that the notebooks would close. (They were really cute when they asked, "But how will the notebook close?" like I hadn't thought of that...) We made a little loop of paper to hold 3 coffee stirrers. I got the colored kind. I would call out two letters and they would hold one straw on the line. Then I'd call out two more and they would hold the other straw on that line. Then we would discuss the relationship. For the first time EVER my kids understood. They didn't get why I was so flippin' happy at the end of class. They're all, "What's the big deal. This is easy."