Language: Chinese
Source: Classical literature

Description

Bāguà (八卦) is usually translated as "Eight Trigrams". They are a set of eight different combinations of three lines, each line appearing in either a broken or unbroken form. The trigrams correspond to a myriad of sets of concepts in Chinese thought, ranging from elements, family ties, anatomy, tastes, natural events, etc.¹

In the center usually appears a tàijí (太極) symbol, literally meaning "grand ultimates", representing the opposing forces  Yīn (陰) and Yáng (陽) from which the Eight Trigrams sprang.

The Bāguà symbol appearing on a sword pommel.
Sold by Mandarion Mansion in 2018.
 

The Eight Trigrams

Each of the trigrams also corresponds to a Daoist immortal:

☰, Qián (乾); Han Xiang Zi (韓湘子), patron of flutists. Practitioner of Daoist magic under Lü Dongbin. Often carries a flute.

☱, Duì (兌); Zhongli Quan (鍾離權), resurrects the dead and transforms stones into silver and gold. Often carries a fan.

☲, Lí (離); Lü Dongbin (呂洞賓), Tang scholar and poët who according to records lives 220 years. Often carries a jiàn sword.

☳, Zhèn (震); Cao Guojiu (曹國舅), patron deity of acting and theatre. Often holds jade tablets or a clapper.

☴, Xùn (巽); He Xiangu (何仙姑), female immortal, ascended heaven during the Tang dynasty. Often carries a healing lotus flower, a sheng instrument, bamboo ladle, or fly-whisk.

☵, Kǎn (坎); Li Tieguai (李鐵拐), benevolent to the poor, sick and needy. Often carries a calabash and/or an iron crutch.

☶, Gèn (艮); Zhang Guolao (張果老), occultist-alchemist. Often carries a fish-drum.

☷, Kūn (坤); Lan Caihe (藍采和), patron of florists and gardeners. Often carries a flower basket.

In the realm of arms and armor

The Bāguà (八卦) or "Eight Trigrams" sometimes feature on weapons. They are seen primarily on Chinese straightswords, jiàn (劍), in the form of inlays in the blade, or on pommels. We often call such swords Daoist jiàn because of their often lavish use of Daoist imagery.

Because the Eight Trigrams are so ubiquitous in Chinese culture, it's often not possible to say anything sensible about their appearance on a weapon. The symbols were used on Qing imperial flags, as well as by rebel groups fighting them. The presence of Eight Trigrams on a saber, therefore, doesn't really tell us much other than that it is very culturally Chinese.

A Daoist set of double swords.
Sold by Mandarion Mansion in 2018.

Another set of Daoist double shortswords.
Sold by Mandarin Mansion in 2019.

A Chinese officer's saber with the eight Trigrams inlaid in the blade.
Sold by Mandarin Mansion in 2018.

Notes
1.  For a comprehensive list, see the Wikipedia article.