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Page Number: 14

10 

CARPENTER v. UNITED STATES 

Opinion of the Court 

[bank] employees in the ordinary course of business.”  Id., 
at 442.  The Court thus concluded that Miller had “take[n] 
the  risk,  in  revealing  his  affairs  to  another,  that  the  in-
formation [would] be conveyed by that person to the Gov-
ernment.”  Id., at 443. 

Three years later, Smith applied the same principles in 
the  context  of  information  conveyed  to  a  telephone  com- 
pany.  The Court ruled that the Government’s use of a pen 
register—a device that recorded the outgoing phone num-
bers  dialed  on  a  landline  telephone—was  not  a  search.
Noting  the  pen  register’s  “limited  capabilities,”  the  Court 
“doubt[ed]  that  people  in  general  entertain  any  actual
expectation  of  privacy  in  the  numbers  they  dial.”    442 
U. S., at 742.  Telephone subscribers know, after all, that
the  numbers  are  used  by  the  telephone  company  “for  a 
variety of legitimate business purposes,” including routing
calls.  Id.,  at  743.  And  at  any  rate,  the  Court  explained, 
such an expectation “is not one that society is prepared to
recognize as reasonable.”  Ibid. (internal quotation marks
omitted).  When  Smith  placed  a  call,  he  “voluntarily  con-
veyed”  the  dialed  numbers  to  the  phone  company  by  “ex-
pos[ing] that information to its equipment in the ordinary 
course of business.”  Id., at 744 (internal quotation marks
omitted).  Once  again,  we  held  that  the  defendant  “as-
sumed  the  risk”  that  the  company’s  records  “would  be 
divulged to police.”  Id., at 745. 

III 
The  question  we  confront  today  is  how  to  apply  the
Fourth  Amendment  to  a  new  phenomenon:  the  ability  to 
chronicle a person’s past movements through the record of 
his cell phone signals.  Such tracking partakes of many of
the  qualities  of  the  GPS  monitoring  we  considered  in 
Jones.  Much  like  GPS  tracking  of  a  vehicle,  cell  phone 
location  information  is  detailed,  encyclopedic,  and  effort-
lessly compiled.