In the same way that stacks collect together your notes, groups can be used to collect together stacks. This gives you much more control over how you organize your library.

For example, if you are learning Spanish and German, you might make one group for each language. In the Spanish group, you could have a stack called Nouns for the Spanish nouns, and another for Spanish verbs. In the German case, you could have this same structure, with stacks for German nouns and verbs.

Groups can be nested as deeply as you like. You can avoid them altogether and just have a simple list of stacks, or you can create a deep hierarchy with groups inside other groups.