Chen Notation - whats the different between "N" and "M" when marking a relationship

Question

This might be a basic question, Im confused. Is there actually a different between M and N when using chen notation. I searched on this but couldn't find an explanation on why two characters if both represent the same thing.

I found : The degree of relationship (cardinality) is represented by characters “1”, “N” or “M” usually placed at the ends of the relationships:

• one-to-one (1:1)
• one-to-many (1:N)
• many-to-one (N:1)
• many-to-many (M:N)

So why not (M:1) (1:M) and (N:N) or (M:M)?

Answer 1

M and N are simply just coefficient. It means there can be any number. (N:M) can be (1:2) , (2:3) .. etc But (N:N) or (M:M) includes (1,1) , (2,2) ,(3,3) ... Basically to represent two different number we use different notations( here "N" and "M"). Yes, there CAN BE a case where N is equals to M.