Endless is derived from End. Period is a type of End. Period is the opposite of Frequency. 
Face is the opposite of Back. Back is a type of Approve. Approve is etymologically related to Improve. 
Endless --DerivedFrom--> End <--IsA-- Period <--Antonym--> Frequency 
Face <--Antonym--> Back --IsA--> Approve --EtymologicallyRelatedTo--> Improve 