  We studied the spectral features of Si II $\lambda\lambda$4130, 5972, 6355
and S II W-trough for a large sample of Type Ia supernovae (SNe Ia). We find
that in NV (Normal-Velocity) subclass of SNe Ia, these features tend to reach a
maximum line strength near maximum light, except for Si II $\lambda$5972.
Spectral features with higher excitation energy, such as S II W-trough, are
relatively weak and have relatively low velocity. SNe Ia with larger
$\Delta$m$_{15}$($B$) tend to have lower velocities especially at phases after
maximum light. NV SNe show a trend of increasing line strength with increasing
$\Delta$m$_{15}$($B$), while 91T/99aa-like SNe show an opposite trend. Near
maximum light, the absorption depth of Si II $\lambda$5972 shows the strongest
correlation with $\Delta$m$_{15}$($B$), while at early times the sum of the
depths of Si II $\lambda\lambda$4130 and 5972 shows the strongest correlation
with $\Delta$m$_{15}$($B$). The overall correlation between velocity and line
strength is positive, but within NV SNe the correlation is negative or
unrelated. In normal SNe Ia, the velocity-difference and depth-ratio of a
longer-wavelength feature to a shorter-wavelength feature tend to increase with
increasing $\Delta$m$_{15}$($B$). These results are mostly well explained with
atomic physics, but some puzzles remain, possibly related to the effects of the
saturation, line competition or other factors.
