The ratio of DA to DB white dwarfs is temperature sensitive. No DBs are found between 45 000 K and 30 000 K, and this exclusion zone is known as the “DB gap”. A working hypothesis to explain the DB gap is proposed. Since the temperatures of the upper and the lower bounds of the DB gap coincide with the effective temperatures where the He II/III and He I/II convection zones show up respectively, convective mixing is suspected to be the cause of the appearance of DBs outside the DB gap. Inversely, chemical separation due to gravitational settling in the convectively stable atmosphere is suspected to be the cause of the presence of the DB gap. According to this scenario, the white dwarfs which will turn back to DBs after the DB gap phase are expected to have a superadiabatic layer which is nonetheless convectively stable due to chemical separation, when they are located near the lower bound of the DB gap. Such white dwarfs are expected to be pulsationally unstable.