In systems of proof based on the English common law tradition, almost all evidence must be sponsored by a witness, who has sworn or solemnly affirmed to tell the truth. The bulk of the law of evidence regulates the types of evidence that may be sought from witnesses and the manner in which the interrogation of witnesses is conducted such as during direct examination and cross-examination of witnesses. Other types of evidentiary rules specify the standards of persuasion (e.g., proof beyond a reasonable doubt) that a trier of fact—whether judge or jury—must apply when it assesses evidence.