There seemed to be two configurations of the towers -- one to achieve the maximum number of people with three in each layer, and one with just a single person in each layer going up from the center with a child on top. The weight of the entire tower seemed to rest on the men just above all the hands, who were always big and pretty burly looking.
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The last major event we attended at the Folklife Festival was the human tower, put on by two teams from Valls -- Colla Joves Xiquets de Valls and Colla Vella dels Xiquets de Valls -- said to be among the best in Catalonia. Participants can be any age or gender, and indeed there were older folks as well as small children taking part. Spectators were told to hold their applause and shouts until the towers were completed because the climbers need to hear the music in order to understand the progress being made and the next steps.
A short VIDEO of the making of one of the towers is at http://vimeo.com/279043268
Red team finishes, posted earlier: