134/413 : Every pairing in SJ (or almost) has his own “number”. Yesung is the 4th oldest member, and Kyuhyun the 13th. So 134 and 413 are the KyuSung’s numbers.

Apple : An important fruit for KyuSung shippers. Everything began in SJ Foresight, when Yesung played an apple and Kyuhyun (who played Snow White) had to "eat" him. Some months later, for SS4 Seoul, Kyuhyun brought apples on stage and fed Yesung... and as Kyuhyun kept bringing apples during the Good Friends performance for every SS4, Yesung started to steal them without Kyuhyun's permission, causing more KyuSung moments.

Confetti : They often wipe confetti away from the other's hair.

FLY : KRY filmed their MV for the song “FLY” right after Kyuhyun’s car accident in april 2011. On many pics taken this day, we can notice that Yesung is very caring toward his dongsaeng, probably because he was so happy to see him again by his side (and we know that Yesung was affected by the first accident in 2007). Even if there isn’t any KyuSung moment in the MV, we know the truth thanks to fancams and pics : Kyuhyun and Yesung were always together during the filming.

GameCloud/GaemCloud/GamerCloud : The name for KyuSung biased. Yesung’s fans are called Clouds, and Kyuhyun’s fans are Gamers, so...GameClouds for KyuSung. 

Hands : It seems like they are obsessed with each other's hands, probably because of the size difference. Kyuhyun likes to tease Yesung about his small hands, and he even grabs them whenever Yesung wants to bite his nails in order to stop that bad habit. There was also a time when they had to compliment each other, and they talked about their hands, Kyuhyun saying Yesung's were cute, and Yesung saying Kyuhyun's were big.

Headphone necklace : The necklace Yesung gave Kyuhyun after his comeback in 2007 (?). Yesung wore almost the same during Don’t Don era (we can find many pics of him wearing it), whereas Kyuhyun wore it only once.  
/!\ not really confirmed yet but I'm still working on it OTL

It Has To Be You :Yesung's solo (march 2010). Kyuhyun was the only member to sing it on stage in december 2010. Then, some months later, they sang it together in a radio show. Kyuhyun really seems to love this song : he sang it to Yesung on the phone once, and started to sing it at Yesung's last Super Show before enlisting, followed by the fans. Those are just examples, of course.

KyuSung Day : We celebrate it on April 13th. Why ? Because it’s related to KyuSung’s numbers (april 13th = 04/13).

KyuSung shipper : Do I have to explain it ? If you think Kyuhyun and Yesung look good together (I mean, it can be about brotherhood, strong friendship or real love), then you are a KyuSung shipper.

Miracle For You (M4U or MFY) : The name of Yesung’s radio show (sept 06 – sept 07). KyuSung moments are precious in M4U, because we can see that they are already very close, and we know that Kyuhyun joined the group only some months before that. He was the only member to call Yesung for his first day on the air. And Yesung replied that he loved him. Then, only two days after, Kyuhyun came at M4U and became the first guest. And he kept coming regularly, until the accident. And I’m pretty sure Yesung recorded his voice message at M4U before posting it on his cyworld.

Phone : It seems like the phone is important in KyuSung's relationship. Kyuhyun called Yesung the first day of his radio show, and he also used to call him and Leeteuk when they were at Kiss The Radio, and Kyuhyun was in China (he even called them after coming back from France in june 2011). And by the way, Kyuhyun called Yesung at midnight for his birthday in 2010. He said he was the first.

SJ Foresight : Kyuhyun and Yesung were MCs of this show with Leeteuk and Eunhyuk (and Shindong, Ryeowook and Donghae replaced them in some episodes). There were many KyuSung moments.

The Night Chicago Died : Another KRY’s song. Even though there are a lot of YeWook moments in those perfs (and also KyuWook, sometimes...), this is the song when you can be sure to see at least one KyuSung interaction. Probably because of this famous perf in SSI, when they hugged. They even hugged for 23 seconds in march 2009. And seriously, what’s better than a KyuSung hug ?

Your Eyes (also known as "Someone like me" or "Naran Saram") : The name of their duet in Bonamana album. They finally sang it at KRY Winter Concerts (9 times), and it was just...too beautiful to be described in words. 

And yes, the turtle is also a KyuSung symbol