The Sixth Amendment guarantees a criminal defendant the right "to be confronted with the witnesses against him." (U.S. Const., 6th Amend.) In Crawford v. Washington (2004) 541 U.S. 36, the Supreme Court held the confrontation clause bars "admission of testimonial statements of a witness who did not appear at trial unless he was unavailable to testify, and the defendant had had a prior opportunity for cross-examination." (Id. at pp. 53-54.) The confrontation clause does not restrict the introduction of out-of-court statements for nonhearsay purposes or the use of prior testimonial statements "when the declarant appears for cross-examination at trial," however. (Id. at p. 59, fn. 9; People v. Cage (2007) 40 Cal.4th 965, 975, fn. 6 (Cage).) After Crawford, courts have "labored to flesh out what it means for a statement to be 'testimonial.'" (Ohio v. Clark (2015) U.S. , 135 S.Ct. 2173, 2179 (Clark).) In Cage, supra, the California Supreme Court delineated its understanding of the characteristics of the category: "We derive several basic principles from Davis v. Washington (2006) 547 U.S. 813. First, as noted above, the confrontation clause is concerned solely with hearsay statements that are testimonial, in that they are out-of-court analogs, in purpose and form, of the testimony given by witnesses at trial. Second, though a statement need not be sworn under oath to be testimonial, it must have occurred under circumstances that imparted, to some degree, the formality and solemnity characteristic of testimony. Third, the statement must have been given and taken primarily for the purpose ascribed to testimony--to establish or prove some past fact for possible use in a criminal trial. Fourth, the primary purpose for which a statement was given and taken is to be determined 'objectively,' considering all the circumstances that might reasonably bear on the intent of the participants in the conversation. Fifth, sufficient formality and solemnity are present when, in a nonemergency situation, one responds to questioning by law enforcement officials, where deliberate falsehoods might be criminal offenses. Sixth, statements elicited by law enforcement officials are not testimonial if the primary purpose in giving and receiving them is to deal with a contemporaneous emergency, rather than to produce evidence about past events for possible use at a criminal trial." (Cage, supra, 40 Cal.4th at p. 984, fns. omitted.) In its most recent pronouncement regarding the parameters of "testimonial" evidence, the California Supreme Court stated: "Although the United States Supreme Court has not settled on a clear definition of what makes a statement testimonial, we have discerned two requirements. First, 'the out-of-court statement must have been made with some degree of formality or solemnity.' (People v. Lopez (2012) 55 Cal.4th 569, 581.) Second, the primary purpose of the statement must 'pertain in some fashion to a criminal prosecution.' (Id. at p. 582; accord, People v. Dungo (2012) 55 Cal.4th 608, 619.)" (People v. Leon (2015) 61 Cal.4th 569, 603.) The United States Supreme Court has also focused on whether, considering all relevant circumstances, including the identity of the interrogator and the informality or solemnity of the questioning, the primary purpose of questioning is to obtain "testimonial evidence against the accused." (Clark, supra, U.S. at p. 135 S.Ct. at pp. 2180-2181.) Because the confrontation clause does not apply to out-of-court statements admitted for nonhearsay purposes, no confrontation violation occurs where an expert witness refers to out-of-court statements upon which he or she relied in forming an opinion because such "basis" evidence is not admitted for its truth. (People v. Gardeley (1996) 14 Cal.4th 605, 619 (Gardeley); People v. Sisneros (2009) 174 Cal.App.4th 142, 153-154; People v. Ramirez (2007) 153 Cal.App.4th 1422, 1427.)