A global phase diagram of bound states in the continuum (BICs) in photonic-crystal slabs is obtained based on a universal classification of BICs by their different Friedrich-Wintgen origins. Previously, BICs in photonic-crystal slabs are classified into three types: single-resonance parametric, symmetry-protected and Friedrich-Wintgen BICs. Here, we show that the single-resonance parametric BICs come from the coupling between the guided resonance (GR) and Fabry-Pérot (FP) modes, and the symmetry-protected BICs from the coupling between two degenerate...