Kanshou and Bakuya

Kanshō and Bakuya (干将・莫耶) are the "married" twin swords that Archer favors during the fifth Holy Grail War. The black sword is Kanshou, the Yang, and the white sword is Bakuya, the Yin.

They are a pair of swords with a strong bond, which will return to its owner even if events cause them to be lost. Their quality as swords is high, but they also serve as ceremonial equipment for sorcery. When both are equipped, magic resistance and physical resistance improve.

It might just be Archer's taste, but words thought to be a charm against evil spirits are carved on the blades.

Divine skills, flawless and firm

Our strength rips the mountains

Our swords split the water

Our names reach the imperial villa

The two of us cannot hold heavens together

—Two rivals, walking their own life.

They were made by the famous craftsman Kanshou in the. It seems he was more crafting for the sake of crafting than for the idea behind the creation. Kanshou's wife, Bakuya, gave her life to melt the metal and create the married swords that represent the Yin and Yang.

Source: based on the weapon status screen, translated by Mirror Moon and the dictionary resource at Jonnobi.

Overedge
See: Shirou vs Black Saber - END (scene translation)

During the Sakura route, Emiya Shirou makes great usage of Kanshou and Bakuya in his fight against Black Saber. A move named Crane Wing Three Ream (鶴翼三連).

But if their legitimate owner -Archer- was doing it, the Overedges would be the result. In the serial Tracing, the third and last pair of Kanshou and Bakuya would assume this shape when reinforced.

The design comes from the animated version of the fight between Archer and Berserker, and was created by Koyama Toshihisa (art director for the Fate/stay night anime); Nasu Kinoko fell in love with the dangerous look of the knives.

The blade is twice as tall as the regular Kanshou & Bakuya, but splinters cover the blade from the ridge to the edge. The original usage of the triple pair of wings is three consecutive X-shaped strikes, without any flying or leaping.

Source: the Character Material encyclopedia entry (p.53)