 FACT: Although Mathilde herself in narration, and her aunt in the  story, say the musical instrument Mathilde plays is the tuba (in  French), yet any euphonium player can easily recognize the musical  instrument shown as a euphonium by seeing its clear images and  listening to its sound, which is a little different yet crucial from  those of tuba. This mistake might be deliberately made, for a real  tuba might be so big for Mathilde that the view could not be taken  appropriately.