  We introduce spiral boundary conditions (SBCs) as a useful tool for handling
the shape of finite-size periodic clusters. Using SBCs, a lattice model for
more than two dimensions can be exactly projected onto a one-dimensional (1D)
periodic chain with translational invariance. Hence, the existing 1D techniques
such as density-matrix renormalization group (DMRG), bosonization,
Jordan-Wigner transformation, etc., can be effectively applied to the projected
1D model. First, we describe the 1D projection scheme for the two-dimensional
(2D) square- and honeycomb-lattice tight-binding models in real and momentum
space. Next, we discuss how the density of states and the ground-state energy
approach their thermodynamic limits. Finally, to demonstrate the utility of
SBCs in DMRG simulations, we estimate the magnitude of staggered magnetization
of the 2D XXZ Heisenberg model as a function of XXZ anisotropy.
