Reel to reel tape question

Hello! I am in the process of transferring my dad's old 3" & 5" reel to reel tape to digital MP3. Question - on many of the tapes, there is a reflective 'shiny' side with the other side being dull. Which side were you supposed to record/play on initially?

It sounds like there is information on both 'sides' of the tape on some of these. Was more curious about this than anything else. Seems like the treated reflective side would have been better to minimize head wear? Thanks to any & all.
The shiny side has no recorded material on it unless 'print through' occurred. The latter condition often comes from the reel being tightly spooled and stored that way for many years. Most likely, the dull side is that which was recorded on. Also, most likely, is that the tapes are quarter inch in width and are four track format. Which means that two tracks (1 & 3) were recorded and then the reel was turned over to record the other tracks (2 & 4). To check, play the reel in one direction and then when it is completely at end, turn the reel over and play it again in the same direction. All this assumes he recorded on a four track machine. If the machine that was used to record the material was 'half track' and not four track, nothing will be heard if you turn over the reel. If the machine you are playing back is four track, and the tapes were recorded in half track, you will still hear the material but one of the channels will be a little less volume.
Reel to reel tapes and even cassette tapes are a mylar plastic with a metalic oxide coating on one side. This gives it the dull appearance. Often referred to as magnetic tape, it is this dull side that has the signal on it. The dull side is the side that contacts the head as the tape goes past to the other reel when recording or playing back. No reel to reel tapes were made with magnetic coating on both sides. Some better tapes were "back coated" to give the pinch roller and capstan better grip for tape handling, but that side was never meant to be in contact with the tape heads.
Hope this helps.