A schematic picture of structure of type Ia supernovae derived by the observations. The ash of the initial sparks is at an offset (yellow). Depending on the viewing direction from which a supernova is observed, a supernova manifests different spectral properties: If viewed from the offset direction, it appears as a "Low-Velocity-Gradient" SN and shows blueshift in late-times. From the opposite direction, a SN is a "High-Velocity-Gradient" SN and shows redshift. Here low- and high-velocity-gradients supernovae are those showing a slow and rapid evolution in spectral features, respectively.