You have one of Amazon’s Kindle ebook readers as well as the Kindle app on an iOS device. When you purchase a Kindle ebook from Amazon, it syncs beautifully between devices—you read to page 212 on one device and when you pick up the other, it asks if you’d like to move to that page on the device you’re currently using.
But suppose you’re picked up an Amazon-compatible ebook from a different site—Project Gutenberg, for example? If you load such a file on a couple of devices you’ll find that they don’t sync. Unless you know the trick that I demonstrate in this video.