The Crawford Court decided the key was whether the evidence was testimonial because of the Sixth Amendment's use of the word "witness." Quoting the dictionary, the Court explained that a witness is one who "bear[s] testimony" and that "testimony" refers to a "solemn declaration or affirmation made for the purpose of establishing some fact.
Nonetheless, in Crawford, the Supreme Court explicitly declined to provide a "comprehensive" definition of "testimonial" evidence (and, thus evidence subject to the requirements of the Confrontation Clause). In Davis v. Washington and its companion case, Hammon v. Indiana, the Court undertook exactly this task. The Court explained what constitutes testimonial hearsay: