  Early polarization observations on Type Ia supernovae (SNe Ia) may reveal the
geometry of supernova ejecta, and then put constraints on their explosion
mechanism and their progenitor model. We performed a literature search of SNe
Ia with polarization measurements and determined the polarization and relative
equivalent width (REW) of Si II 635.5-nm absorption feature at -5 days after
the maximum light. We found that either the distribution of observed
polarization degree is bimodal, i.e. the broad line SNe Ia have systematically
higher polarization than all other SNe Ia, or all kind of SNe Ia share the same
polarization sequence, i.e. the polarization of Si II 635.5-nm absorption
feature increases with the REW. We also discussed the potential meaning of the
discovery on the explosion mechanism and progenitor model of SNe Ia.
