Hinge Caliper turns the iPhone Duo's precision hinge into a measuring tool. Close your Duo on any object and it computes the thickness from the hinge angle — then vibrates and speaks the measurement aloud, so you don't need to see the screen. Includes a 0–180° protractor dial, outer-display mirroring while you work, plus on-screen rulers and bubble levels that run on any iPhone. No ads, no data collection, works offline. $0.99.