Corners of the kaba
There are four corners of Kaba. Each corner is called a Rukn.  They are: (1) Rukn aswad  (2) Rukn yamani  (3)  Rukn iraqi (4) Rukn shami.
Rukn aswad has two glories! (1)This is constructed on the foundations laid down by Hadrat Ibrahim alaihis salaam and (2) Hajr aswad (the black stone) has been fixed in this corner, hence this is kissed by either hands or by lips.
Rukn yamani has one glory. This has also been constructed over the foundation laid by Hadrat Ibrahim alaihis salaam. Hence, touching this Rukn by hand and kissing the hand is a sunnah. It is better not to kiss by lips. (Mirqata ulmafaateeh, Book 9, Page 38)
The remaining other two  that is Rukn iraqi and Rukn shami have not been having such glories, because they are in between the earlier structure of  Kaba.  The holy hateem  too is a part of Kaba, hence it is not a sunnah to kiss the holy hateem.(Mir’aatlumanajeeh, Volume 4, Page 183)