I can see how they seem a bit odd here; as used in the context of the much larger set introduced in Session 2.4C (“Maintaining the Ravel”), we found that the double lines helped kids attend to place value. Teachers are of course free to adapt, but if they explain the double lines to the kids, they might find them useful.
Reply To: Unit 2; Session 2.3,C: From Personal to Collective Understanding
Forming a community of practice