One folding style with several printable designs (some takes lots of ink). Creator also offers OpenOffice.org Draw templates (Creative-Commons license) to create your own Letterfu design.
S. Horras, M. Hoffmann, и T. Niesen. Intelligente Vernetzung in der Fabrik: Industrie 4.0 Umsetzungsbeispiele für die Praxis, Fraunhofer Verlag, Stuttgart, (2015)
O. Gassmann, K. Frankenberger, и M. Csik. Working Paper, Institute of Technology Management of the University St. Gallen (ITEM-HSG), St. Gallen, Switzerland, (августа 2013)