on: a string of finite length
is: that standing wave of the string which has the greatest possible wavelength (and hence the lowest possible frequency, known as the fundamental frequency). [P5.6, P5.7]
is exemplified: for a string fixed at both ends with linear mass density 
and under a tension 
by the standing wave of wavelength 
which has (fundamental) frequency 
may be more generally applied: to other oscillatory systems that exhibit standing waves.
See harmonics.
Copyright 1997, The Open University