Sqrt(2/1)
sqrt(2/1), the semioctave or perfect median, is an important radical interval of exactly 600 cents. It appears in hemipyth as one of the generators, alongside sqrt(3/2).
In temperaments
Many temperaments equate a just interval (or more accurately, a pair of just intervals) to the semioctave; among the most common to be merged this way are 7/5 and 10/7 (which differ by 50/49), 17/12 and 24/17 (which differ by 289/288), and 99/70 and 140/99 (which differ by 9801/9800). The semioctave appears in every even equal temperament.
Name
"Semioctave" corresponds to its construction as the logarithmic division of an octave into 2. "Median" is a term for a tritone as a distinct interval degree borrowed from Leriendil's parlance.