A perfect ruler, also called a complete ruler, is type of ruler which has 
distinct marks placed at integer distances from the end such that the distances between
marks can be used to uniquely measure all the distances 1, 2, 3, 4, ... up
to some maximum distance 
.
For example, the perfect difference set 
gives 0, 1, 4, 6, which can
be used to measure 
,
, 
, 
, 
, 
, thus giving 6 distances using only four marks, as illustrated
above (Gardner 1983, Fig. 91, and pp. 153-154).
No perfect Golomb ruler, i.e., a ruler that uniquely measures distances up to its length, exists for five or more marks (Golomb 1972; Gardner 1983, p. 154).
