More than 20 results in tab sizes that are only practical for numbers or symbols. Multiple banks can provide all the copy space you desire. Partial and mixed banks can provide clever solutions to indexing challenges. Creative collating can add further versatility. Each consecutive tab is progressively larger than the preceeding tab so that only the last tab is visible from the back.
Tabs are generally square on one end.
This is a type of step indexing that works in reverse. Each tab overlays the tab which follows it.