xxxHOLIC's realm of "Japan" is our modern day Japan, thus making the xxxHOLIC universe, ours. It's further explained in xxxHOLIC about the connections between the characters of TSUBASA RESERVoir CHRoNiCLE and their former counterparts, such as in Cardcaptor Sakura.


xxxHOLiC in Modern-day Japan.
Aside from the fact that Kurogane and Fay are exclusive to TSUBASA RESERVoir CHRoNiCLE right now, Sakura and Syaoran in TSUBASA RESERVoir CHRoNiCLE are, in truth, the actual Sakura and Syaoran from Cardcaptor Sakura. Sounds confusing, huh? Yuko further explains in Volume 2 of xxxHOLIC that, although they are leading different lives, Sakura and Syaoran from Clow share the same souls of the ones in Yuko's dimension (Japan; Sakura and Syaoran from Tomoeda). So, if it's put into theory, the storylines of CCS and TRC are happening at the same time. If you really think about it, it could even go up to the point that TRC-Sakura lost her wings at the same time CCS-Sakura opened the Clow Book! Pretty freaky, isn't it? Let's continue.


Yuko and Clow with the Mokonas - Soel & Larg.
As hinted at in Volume 1 of xxxHOLIC, Yuko and Clow are old colleagues, and teamed up to create the Mokonas, in preparation for TRC-Syaoran and Sakura's journey. This is also the same way that Clow prepared the Clow Staff for CCS-Sakura before his death. Confusingly enough, Yuko also states that Syaoran and Sakura are both of Clow's blood relations. While it's hard to understand if you're a CCS-only or TRC-only fan, it's actually true - CCS-Syaoran hails from Clow's Hong Kong family, while TRC-Sakura is his daughter in the Kingdom of Clow. An even weirder fact is that the incarnations of Fujitaka - who has a special connection to Clow Reed in CCS - is father to both Syaoran and Sakura in different realms! This also means that Clow exists among different dimensions, as we all do, according to Yuko. But, Clow was aware of his other presences in different worlds, and thus could team up with Yuko in order to prepare for everything beforehand.

CLAMP is reknowned for their horribly confusing storylines, which is why it helps to have a solid base of background knowledge on their mangas to help out. Some previous CLAMP mangas worked into TRC and some of xxxHOLIC are X/1999, Tokyo Babylon, RG Veda, Card Captor Sakura and Chobits.