Three volts is to divide the hottest period in a year into three periods, namely, head, two volts and three volts. Counting from the third Geng day after the summer solstice, the first sequence is ten days, which is called Toufu or Chufu, the second sequence is ten days, which is called Zhongfu or Erfu. Counting from the first Geng day after beginning of autumn, the next sequence is ten days, which is called Fufu or Sanfu.

The number of days in the ambush varies from year to year, sometimes 30 days, and sometimes 40 days. Some years are long and some years are short in dog days, all because of the difference in the number of days in the middle of the dog days.

The time of the head and the end is certain, and it is ten days every year. However, the midriff is determined according to the head and the end, and the number of days in midriff is not so fixed. When there are four Geng days between the summer solstice and beginning of autumn, the midsection is 10 days; When there are five Geng days, it will be 20 days in the middle.