Idghaam Naaqis[]
When Waw or Yaa is the first letter for the sencond word the Idghaam (merging) is incomplete. The is that the Noon Saakinah(also the Noon Saakinah of the Tanween), has a Ghunna, the Yaa and Waw don't. this is the reason why we do not see a Shadda over the Yaa or Waw in the majority of Qur'aanic writings when there is Idghaam.