The cover of this idiom book demonstrates what 画竜点睛 literally means

 

 

画竜点睛 is a four-character idiom in both Chinese and Japanese which means “to add finishing touches” and “completing by executing the final, critical step”. Here they are in their Traditional Chinese characters and Japanese kanji forms:

畫龍點睛
Bopomofo: ㄏㄨㄚˋ ㄌㄨㄥˊ ㄉㄧㄢˇ ㄐㄧㄥ
Hanyu Pinyin: hua4 long2 dian3 jing1

画竜点睛
がりゅうてんせい

If you break this idiom down character-by-character, what you get is “draw dragon dot eyes”. So how does the relate to “finishing touches”???

According to the Taiwan Ministry of Education, this idiom means:
畫了龍後,為其點上眼睛,龍便乘雲飛去。比喻繪畫、作文時在最重要之處加上一筆,使全體更加生動傳神。
“After one draws (畫/画) a dragon (龍/竜), one draws (點/点) the eyes (睛), and the dragon then flies above the clouds. In terms of drawing and writing, this means adding a stroke to the most important part, making the work more vivid or lifelike.”
NOTE: in Chinese, 點 or 点 means to touch the paper lightly with pen, producing a dot (the eyes)

But why eyes??
According to legend, a famous ancient Chinese artist (張僧繇- zhang1 seng1 you2, ちょうそうよう) had painted dragons without eyes on the wall of a temple. When asked why he wouldn’t paint in the eyes, he replied “the eyes of a dragon are where its spirit is. All other parts are merely parts of the physical body. Once I paint eyes, the dragon will fly away, soaring above clouds”. Unsatisfied with this explanation, people challenged the artist’s claim. Reluctantly, 張僧繇 drew in the eyes of one dragon. Suddenly, a strong gust of wind blew forth and a bolt of lightning flashed and, just as the artist had warned, a real dragon emerged from the wall and flew into the sky.

Drawing the eyes on a dragon gives it the ability to soar in the sky just as giving a piece of work the final, culminating touches lend to the work its spirit and soul.

In Japanese, 画竜点睛 can be used in the construction: 画竜点睛を欠く(がりゅうてんせいをかく)which means “to lack the most crucial part”.

Links:
http://dict.idioms.moe.edu.tw/pho/cy/cy00383.htm
http://www.epochtimes.com/b5/6/3/26/n1267171.htm

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