Though Chinese has words for the four cardinal directions, Chinese mariners and astronomers/astrologers preferred using the 12 directions of the Earthly Branches, which is somewhat similar to the modern-day practice of English-speaking pilots using o'clock for directions.
Since 12 points were not enough for sailing, 12 midpoints were added.
Instead of combining two adjacent direction names, they assigned new names:
For the four diagonal directions, appropriate trigram names of I Ching were used.

For the rest, the Heavenly Stems (1-4, 7-10) were used.
According to the Five Elements theory, east is assigned to wood, and the Stems of wood are 甲 (jiǎ) and 乙 (yǐ).
Thus, they were assigned clockwise to the two adjacent points of the east.

The 24 directions are:
Advanced mariners such as Zheng He used 48-point compasses.
 An additional midpoint was called by a combination of its two closest basic directions, such as 丙午 (bǐngwǔ) for the direction of 172.5°, the midpoint between 丙 (bǐng), 165°, and 午 (wǔ), 180°.
Though Chinese has words for the four cardinal directions, Chinese mariners and astronomers/astrologers preferred using the 12 directions of the Earthly Branches, which is somewhat similar to the modern-day practice of English-speaking pilots using o'clock for directions. Since 12 points were not enough for sailing, 12 midpoints were added. Instead of combining two adjacent direction names, they assigned new names:
For the four diagonal directions, appropriate trigram names of I Ching were used.
For the rest, the Heavenly Stems (1-4, 7-10) were used. According to the Five Elements theory, east is assigned to wood, and the Stems of wood are 甲 (jiǎ) and 乙 (yǐ). Thus, they were assigned clockwise to the two adjacent points of the east.
The 24 directions are:
Advanced mariners such as Zheng He used 48-point compasses.  An additional midpoint was called by a combination of its two closest basic directions, such as 丙午 (bǐngwǔ) for the direction of 172.5°, the midpoint between 丙 (bǐng), 165°, and 午 (wǔ), 180°.
Earthly Branches