Grandpa writes about giving a note to Grandma's doorman on 9/11/01 and how it took a while for her to agree to see him again.
After a few days, she lets him into their guest room (a Nothing space).
Grandpa on the plane -> He called Grandma and writes about 4,000 numbers to communicate
A few days later, the World Trade Center was attacked.
Grandpa writes that on that day he lost everything for the second time.
Grandpa asks if he can see Oskar, but Grandma says no.
He watches Oskar anyway, through the keyhole in his bedroom door.
One day, Grandma tells Grandpa about Thomas, "Not our son, my son" (14.1), she emphasizes.
She tells him that she gave him the one letter he ever sent (this must have been the one marked with red pen) when he wanted to find his own father --> has found, but he pretend to be a journalist.
When Oskar starts his search for the Blacks, Grandpa follows him around
Back to the point where Oskar meets Grandpa (the renter):
That night, Grandma and Grandpa have sex again, and later, Oskar throws pebbles at the window, meets Grandpa outside, and tells him about his idea to dig up the grave.