空凳 (empty stool / kōngdèng) is a reference to the empty chair between Thorbjørn Jagland and Kaci Kullmann Five during the awarding of the 2010 Nobel Peace Prize…
Why it is blocked: …an award that the recipient, Liu Xiaobo, was unable to receive because he was in prison. One of the iconic images of the evening was head of the Nobel Committee Thorbjørn Jagland placing the Nobel medal onto the vacant chair. Hong Kong media and netizens used the phrase to recognize Liu’s situation and accomplishment. China’s most recent Nobel winner, novelist and short story writer Mo Yan for literature, made headlines when he called for the release of Liu.
Coincidentally, the Hong Kong musician, Danny Summer (夏韶聲), the “father of Hong Kong rock-and-roll” (not to be confused with Cui Jian, “the father of Chinese rock”) and writer of the classic Tiananmen Square tribute song “Mama, I Didn’t Do Anything Wrong,” had penned an unrelated ballad to his dead father also entitled 空凳 in 1985.
(空凳 was blocked at the end of 2011 and is now currently unblocked.)

