3Teach with confidence, using the world's favorite English course. Interchange is a four-level, American English course that has been used by over 50 million students worldwide. This Intro Level Student's Book includes 16 units, eight progress checks, and a Grammar Plus section to ensure that students receive all of the practice they need. Inside each book is a single-use code for online self-study material. This gives access to videos and hundreds of automatically graded exercises, and works on PCs and Macs.