Relative Density (RD) is a way to measure how crowded a stand is based on the given tree diameters (specifically, the quadratic mean diameter (QMD)). Quadratic mean diameter (QMD) is the diameter based on the stand’s basal area.  It is not just the average diameter of trees in the stand.  The QMD = the square root of the tree diameters (dbh) squared divided by the total number of trees; this calculation ties the diameter more closely to the basal area of each stand.  QMD à     
  Basal area (BA) is the area on the ground occupied by each tree in a stand on a given area (i.e. acre)   

Relative Density Example:  On a variable plot using a 40-factor prism, seven trees are designated as “in” trees.  Those seven trees multiplied by 40 yields a basal area of 280 ft². Calculation:  a stand with QMD = 14.5 inches and BA = 280 ft² yields: RD à  280 ft² = 73.5                              √14.5 in  

Designation by Description Designation by description (DxD) is a technique used to indicate trees to be cut or retained using a combination of species, diameter and stocking criteria (spacing, basal area or trees per acre) in lieu of marking trees with paint.  The concept is based on leaving the largest tree and cutting other smaller trees within a given distance of that tree.  The largest tree is determined by measuring the basal diameter at a height of 4 inches from the ground.  All trees not restricted by some predetermined criteria within a given distance would be cut.  The next largest tree is then identified and the process repeated.