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18 

OHIO v. AMERICAN EXPRESS CO. 

BREYER, J., dissenting 

and  vice  versa.    See  ibid.    But  this,  too,  is  commonplace.
Consider,  again,  a  farmers’  market.  The  more  farmers 
that  participate  (within  physical  and  esthetic  limits),  the 
more  customers  the  market  will  likely  attract,  and  vice 
versa.  So too with travel agents: the more airlines whose 
tickets a travel agent sells, the more potential passengers
will  likely  use  that  travel  agent,  and  the  more  potential
passengers  that  use  the  travel  agent,  the  easier  it  will 
likely  be  to  convince  airlines  to  sell  through  the  travel 
agent.  And  so  forth.    Nothing  in  antitrust  law,  to  my
knowledge, suggests that a court, when presented with an 
agreement  that  restricts  competition  in  any  one  of  the 
markets my examples suggest, should abandon traditional 
market-definition  approaches  and  include  in  the  relevant 
market services that are complements, not substitutes, of 
the restrained good.  See supra, at 10–11. 

2 
To  justify  special  treatment  for  “two-sided  transaction
platforms,”  the  majority  relies  on  the  Court’s  decision  in 
United  States  v.  Grinnell  Corp.,  384  U. S.  563,  571–572 
(1966).  In Grinnell, the Court treated as a single market
several  different  “central  station  services,”  including
burglar alarm services and fire alarm services.  Id., at 571. 
It  did  so  even  though,  for  consumers,  “burglar  alarm  ser­
vices  are  not  interchangeable  with  fire  alarm  services.” 
Id., at 572.  But that is because, for producers, the services 
were indeed interchangeable: A company that offered one 
could  easily  offer  the  other,  because  they  all  involve  “a 
single  basic  service—the  protection  of  property  through 
use of a central service station.”  Ibid.  Thus, the “commer­
cial realit[y]” that the Grinnell Court relied on, ibid., was 
that the services being grouped were what economists call
“producer  substitutes.”    See  2B  Areeda  &  Hovenkamp 
¶561,  at  378.  And  the  law  is  clear  that  “two  products
produced interchangeably from the same production facili­