Dear Christoph, Thanks a lot for a very thorough and clear explanation. I think I even understood most of it. :) I do not think there is anything wrong with the current treatment of sublattices, once it is properly understood. It probably boils down to personal taste. Just like thinking in terms of lattice with polyatomic basis vs a collection of monoatomic lattices. Re: "So what we could do instead is warn whenever a lattice object is created explicitly that is identical to an already existing one, but unnamed." Every little helps so why not. On the other hand I suspect the message itself will not be sufficiently clear for someone without the good understanding of how Kwant is representing a physical system/model ... cough ... a Fortran programmer like yours truly. A useful thing would be to actually check for site overlap during attachment of the lead but this might violate your philosophy of decoupling site from its position. I do see where you are coming from. The TB Hamiltonian is indeed well described by the graph structure -- set of nodes plus connections -- with positions important only in the setup phase. On the other hand lead attachment is the setup phase... I guess it all depends on what would be the wording of the warning. A simple statement of there being two identical objects would be sufficient for an experienced user, who might not need it anyway, but useless for a newbie. I have to admit I struggle a bit with the sophistication and the abstraction level on which Kwant operate. But then I never properly learned the whole OO programming paradigm etc... To summarize a warning would be nice, but perhaps it could point to specific problems, at least aas example? In any case thanks again for your answer. It was certainly helpful for me. Regards MZ