[1] The pine, cypress, and fir are linked by being similar evergreens.
[2] Some examples of Chinese cultural symbology can be found in the poetry of Six Dynasties poet Tao Yuanming (365?–427).
According to Yeh Chia-ying, one of Tao's most frequently used metaphors is that of the pine tree: symbol of ability to withstand the adversity of cold winds, to withstand the adversity of frosts, nevertheless retaining its own essential character: a situation which can be compared with that of certain persons of metaphorically similar character.
In Chinese cultural symbology, this motif is considered suitable to send to those who are poor or lonely.
[6] The three friends of winter motif is known as early as the Song dynasty work the Jishanji (霽山集), or, in English, the "Clear Mountain Collection" by Lin Jingxi (林景熙), who lived 1242–1310.