This is a clever idea - it gives a much larger viewing area than most mirrors.

You can tape the three sheets together, fold them up, and insert them into a soundhole. (Works with most guitars - won't fit inside the smallest parlour guitars.)

When unfolded inside the guitar, it gives you a 7-1/2" x 12" flat viewing surface, for the big picture of the guitar's interior. Made of unbreakable mirrored acrylic, it frees both your hands for the job. Supplied untaped.