What do the following letter pairs have in common?

DO
BO
LA
CO
TO
SO

HINTS:
#1: They can all be followed by the same letters.
#2: They can all be followed by the same three letters.

Answer:

They can all be followed by UGH:

DOUGH
BOUGH
LAUGH 
COUGH
TOUGH
SOUGH 

NOTE from JA: This puzzle was published in GAMES/WORLD OF PUZZLES - April, 2015 - using the letter pairs BO, CO, DO, LA, RO, TO