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The tribe of Judah were the descendants of Judah, son of Jacob and one of the Twelve tribes of Israel.
The gemstone of the tribe of Judah was the garnet, which appeared first in the second row in the Breastplate of Judgment. The flag of Judah depicted a lion, the tribe's special symbol.
During the forty years of wandering in the wilderness, the tribe of Judah would always encamp to the east of the Tabernacle and would march in the first rank, presumably to the left of the tribes of Issachar and Zebulun, which would encamp and march next to Judah.
The inheritance of Judah turned out to be a fairly sizeable amount of land in the southern portion of the promised land, a good position to be in since invaders really came from the desert to the south.
The Messiah was also to come from the line of Judah. King David was from the line of Judah and Jesus was from the line of David.
When the kingdom of Israel split in two after the death of Solomon and under the harsh terms set up by his son Rehoboam, it was the tribe of Judah along with Benjamin that made up the southern kingdom, Judah. A descendant of David, and therefore Judah, continued to rule on the thrown until its fall several hundred years later.