I have a question I'm a little embarrassed to be asking: what is meant in condensed matter when someone describes a system with "open boundary conditions," say in one-dimension for simplicity? I am comfortable with the statement of fixed (Dirichlet) or free (von Neumann) boundary conditions, as well as periodic of course. But I also see the terms open/closed, which I want to be identical with free/fixed respectively, but I can't find a simple source which just states a definition. If it helps, I'm asking because I would like to set up an analytic calculation to compare with a numerical DMRG paper which simply states that they take open boundaries. Is there a simple definition somewhere to tell me what this means? Maybe a better understanding of DMRG would help me. I would be fine with a definition in terms of lattice models, though I'm interested in taking the continuum limit quickly. I saw one source which mentioned imagining the system on an infinite lattice but simply "turning off" couplings at the edges of the system. This seems weird to me; the eigenfunctions of (say) the Laplacian with these boundary conditions are Dirichlet (sine) in the continuum limit, but I expect "open" to be used in a thermodynamic sense, where the system can exchange energy through its boundaries.