Royal Cubit = 7 palms x 4 fingers (4 digits) = 28 parts 

"The royal cubit was 523 to 525 mm (20.6 to 20.64 inches) in lengthArnold 1991: 296, and was subdivided into 7 palms of 4 digits (4 fingers) each, for a 28-part measure in total."

I added the explanation that the palm is divided into 4 fingers or 4 digits of equal length. - p. 31 Sacred Geometry by Stephen Skinner (Sterling Publishing, 2006) - Brad Watson, Miami, FL 64.136.26.22