CONCERTINA

[noun] [C] - a small six-sided musical instrument, with a folding middle part, which is played by pushing the instrument inwards with the hands and pressing buttonsSome folk dances are performed to the music of a concertina.(UK) A concertina file (US accordion file or folding file, ANZ expanding file) is a box-shaped cardboard container used for storing documents. It has many small folds along two of its sides that allow it to be made larger or smaller as necessary.See picture: Files