When you close an app with the Home button, it continues to run in the background and still uses memory, which might be lacking for other apps.
Therefore, especially before performances, it is highly recommended to completely close all these apps, including SongBook+, and then restart only SongBook+ so that as much free memory as possible is available for this app.
Double-click the “Home” button on your iPad. On an iPad or iPhone without a "Home" button, swipe your finger from the bottom into the screen, stop briefly, and then release.
Swipe the app you want to close upward and out of the screen.
If you don't see the app you want to close, you can scroll until the desired app is visible.
