  We show a flaw in the photonic band structure which forms the basis of our
understanding of crystal momentum in photonic systems. We investigate the
origin of this flaw and propose a continuous-folded band structure as the
rectification. Based on the proposed continuous-folded photonic band structure,
a bound state in the continuum (BIC) is actually below the light line, similar
to the conventional guided modes. We present the condition for the boundedness
of photonic modes and verify the effectiveness of our condition via two
photonic crystal slabs with resonance-trapped BIC and symmetry-protected BIC,
respectively. In the second case, our condition of boundedness correctly
indicates a radiative mode, which satisfies the condition of symmetry-protected
BIC.
