Defining what is an old instruments is complex. In a general way, we can distinguish two categories of old instruments. On one hand, there are the instruments which are not or barely in use nowadays. Due to a period of abandonment, those instruments are representative of an epoch different from ours and can be relatively easily dated. On the other hand, there are the instruments which are still being played despite having been made a few centuries ago. Time and use have usually given them a patina, so they are perceived as old in terms of visual aspect, but they have usually been modified as well and so they can be perceived as contemporary in terms of sound. Do we understand an old instrument by hearing it? Do we actually even need to hear it to understand it? This article is meant to provide some thoughts on these questions, to highlight the links between sight and hearing in our perception of a musical instrument and to illustrate how documentation and scientific knowledge can influence this perception 1 .