The twelve summoned at that time took an oath that they would not give false evidence, nor knowingly conceal the truth; and by knowledge, says Glanvilb was meant what they had seen or heard by trustworthy information, and this shows most clearly how entirely they were looked upon as mere witnesses, and how different the idea of their duties then was from what it is now
9 Thus we see that jurors founded their verdict on their personal knowledge of the facts in dispute, without hearing the evidence of witnesses in court. .
There came a significant change whereby the jury ceasing to be witnesses themselves and gave verdict upon the evidence brought before them at the trials when there was an exception in the case of deeds in which persons were named as witnessing the grant or other matter testified by the deed.10 As we can see, the role of jury continued to change over a hundred of years since 12th century from initially mere recognitors, up to the time when they finally became separated from the witnesses, and gave their verdict, not from their own previous knowledge of the disputed facts but from a consideration of the evidence which was brought before them. It was by about the 15th century when it begun to assume the role of the independent assessor of fact in a case, and it is from this that its modern-day role has developed. By examining the historical changes of the role of jury, indirectly, we can see it changed to adapt to the society requirement at that time and so we can understand better the role of jury in English legal system.
Unlike the present-day jury, the jurors prior to 17th century were just mere recognitors, if they gave a wrong verdict they must usually have been guilty of perjury. Hence, at common law, they became liable to the writ of attaint. In the 16th century, examples are to be found of various prerogative courts undertaking to punish jury who found verdicts manifestly against the evidence.