Figure 4
From: Reconciliation and local gene tree rearrangement can be of mutual profit
A gene tree G with a weak edge (w,v) selected for an NNI. v is connected to two subtrees G
c
and G
d
, while w is connected to v and to the subtree G
b
. Performing an NNI operation around (w,v) means interchanging subtree G
b
with either G
c
or G
d
, leading to trees and 
respectively.