Flashcard organizer, classifying cards in 5 levels from unknown to known. The color of the cards represents a characteristic of the card, typically the duration since the last positive test: the card turns darker over time, indicating the need to refresh it from time to time. This is very useful when e.g. learning Japanese kanji where a large set of characters must be kept in memory. Other color modes provide more detailed means to highlight cards prominently that must be refreshed, or represent the numbers of successes or failures.

The app provides tools for testing and learning. Testing is used to assess the proficiency state of the cards, by moving it to the right side (known) or left side (unknown) with the arrow buttons, or by directly dragging the card on the screen. Learning is used to iterate through a short, growing list of cards until the new words are mastered sufficiently for the moment. The next testing session will show if the words stick.

Every card has up to four attributes, which can be aligned in specific sequences in the testing and learning tools, to individually train specific skills. In addition to the displayed text, the attributes can be presented in a read aloud form with the text-to-speech function of iOS, with fully customized voices in all supported languages.

A growing archive of wordlists is available on the homepage (stackz.com), or new wordlists can be created with the free Stackz List Editor on Windows.