As we can bid the 35 kind of contract in its order during auction, there are 2 important special bids that can be made in addition of those 35 bids: Double (X) and Redouble (XX). And because it’s special bids, they have special requirement and special purpose as well.
This is really happening to me on a supervised session on my club: I open 1♦ as natural 12-21HCP promising at least 3 cards Diamond. LHO passes and partner bids Double !! Apparently the person opposite me has doubleton Diamond, 4-4 majors and 3 cards club and wanting me to be 2nd suit if any… He is not really beginner’s beginner as has been playing bridge regularly for more than 1 years, but it highlite one simple fact that probably the understanding of this Double bid (and even the rarer Redouble) is not really well explained to the beginner as one of the bridge basic.