Although arbitration rules give rarely any indication upon whom the burden of proof rests, this issue is an essential one in arbitration. Rules on the burden of proof allow not so much to determine the party who should bear the duty of producing evidence proof, but especially the party who should bear the risk in case there is any doubt, i.e. which party shall not prevail in case it cannot bring forward sufficient evidence in support of its case. In arbitration, the parties have not only the duty to prove the facts but also the duty to bring evidence on the applicability and the content of the legal rules on which they rely in support of their allegations. Albeit they need only establish the facts which are out of the ordinary or unusual.