The Ninth

What is it about the number nine when it comes to symphonies?

Here is the fourth movement of Antonin Dvorak's ninth symphony:


You may not know this tune(s) but you most likely know of the second movement:


Dvorak's #9 is also known as the New World Sympony as Dvorak composed and debuted the symphony here in the USA.

Here is the finale of another #9:


If you listen the below you will hear a very recognizable melody:
Yeah, that tune you hear in a number of adverts is from Beethoven's ninthy symphony.

Another famous ninth symphony is from Schubert:
This is also a great symphony. What is it about number nine?