In actuality, the detailed shape of the vibrating string at some particular instant in time
(usually conveniently taken to be t = 0) – known as the “initial conditions” (not to be confused
with {spatial} boundary conditions!) actually determines/defines the relative amount(s) of the
allowed sin ωnt vs. cos ωnt type solutions – because this simply specifies, for each eigen-mode
what its phase is/how far it is along in its oscillation cycle at time t = 0. Temporal phase
information is “encrypted” into the allowed eigen-solutions, Tn(t) of the temporal Helmholtz
equation as follows: