$x<y$ means that $x$ is less than $y$, but is it only meaningful/valid if $x$ and $y$ are elements of the same set?
$x<y$ iff $(x,y)\in<$ right?
$(x,y)\in<$ tells that the ordered pair $(x,y)$ is an element of the relation $<$, but what does this ACTUALLY mean?
For example if I have $xRy$, then the meaning of this depends on the definition of the $R$ relation, for example it could mean $x$ is equal to $y$, or $x + 69 = y$. $R$ could very well be defined with $x$ and $y$ being elements of different sets.
Does the same apply for the $<$ symbol?