The tape measure may consist of a relatively large tape blade. To accommodate a large blade the tape measure housing is formed as a hollow annulus having an open central core where the blade is wound about the core of the annular housing. Because the housing has a central opening it can be easily carried...http://www.google.com.tw/patents/US7559154?utm_source=gb-gplus-share專利 US7559154 - Tape measure
