When led to a seat, any worthy candidate beheld his name suddenly appear on its back, in golden letters, which vanished only at his death, or when he became unworthy to occupy a seat at the Round Table.

Besides, on one side of Arthur's throne was the Siege Perilous, which none could occupy, under penalty of destruction, save the knight destined to achieve the Holy Grail.
We are informed that Arthur sent his best friend and most accomplished knight, Launcelot, to escort Guinevere to Caerleon on Usk, where the wedding and first session of the Round Table were to take place on the self-same day.

It seems that, when this Launcelot was a babe, his parents had to flee from a burning home.

Overcome by sorrow and wounds, the poor father soon sank dying beside the road, and, while the mother was closing his eyes, the Lady of the Lake suddenly rose from her watery home, seized the babe, and plunged back with him into its depths.

The widowed and bereft woman therefore entered a convent, where she was known as the Lady of Sorrows, for little did she suspect her son was being trained by Pellias--husband of the Lady of the Lake--to become the most famous knight of the Round Table.