Hello everyone.
I am having a problem with the compound words. With the letter "s" to be exact.
For example: "die Kundennummer". It contains two words : "die Kunden" , "die Nummer".
On the other hand, there is: "die Vertragsnummer". It contains "der Vertrag" , "die Nummer" , and the "s" between them.
My question is: When do we put this "s" in compound words and when we don't?