Open the app and your entire display becomes a button—tap anywhere, that’s one. Tap again, two. You can’t miss, but if you want, you can control a little bit of how this works. Swipe to the left to name the current tally (you can keep separate tallies for whatever you need), adjust the number that Tally increments with each tap (one tap = 2, or 20, etc.), and specify whether Tally counts up or down.
Check out Tally, it’s a great app that just got an iOS 7 update.
