The restoration and maintenance of museum ships presents problems for historians who are asked for advice, and the results periodically generate some controversy. For instance, the rigging of sailing ships has almost never survived, and so the rigging plan must be reconstructed from various sources. Studying the ships also allows historians to analyze how life on and operation of the ships took place. Numerous scientific papers have been written on ship restoration and maintenance, and international conferences are held discussing the latest developments. Some years ago, the Barcelona Charter was signed by a variety of international owner organizations of traditional vessels, and provides certain accepted minimum criteria for the restoration and operation of traditional watercraft still in operation.