  Using the largest spectroscopic dataset of stripped-envelope core-collapse
supernovae (stripped SNe), we present a systematic investigation of spectral
properties of Type IIb SNe (SNe IIb), Type Ib SNe (SNe Ib), and Type Ic SNe
(SNe Ic). Prior studies have been based on individual objects or small samples.
Here, we analyze 227 spectra of 14 SNe IIb, 258 spectra of 21 SNe Ib, and 207
spectra of 17 SNe Ic based on the stripped SN dataset of Modjaz et al. (2014)
and other published spectra of individual SNe. Each SN in our sample has a
secure spectroscopic ID, a date of $V$-band maximum light, and multiple spectra
at different phases. We analyze these spectra as a function of subtype and
phase in order to improve the SN identification scheme and constrain the
progenitors of different kinds of stripped SNe. By comparing spectra of SNe IIb
with those of SNe Ib, we find that the strength of H$\alpha$ can be used to
quantitatively differentiate between these two subtypes at all epochs.
Moreover, we find a continuum in observational properties between SNe IIb and
Ib. We address the question of hidden He in SNe Ic by comparing our
observations with predictions from various models that either include hidden He
or in which He has been burnt. Our results favor the He-free progenitor models
for SNe Ic. Finally, we construct continuum-divided average spectra as a
function of subtype and phase to quantify the spectral diversity of the
different types of stripped SNe.
