The presentation is arranged as follows. Section 1 overviews the status and challenging issues of hydrogen bond relevance. Section 2 introduces the basic regulations of hydrogen bond cooperativity and polarizability (HBCP) for the coupling system and the bond order-length-strength correlation and nonbonding electron polarization (BOSL-NEP) for the effect of atomic/molecular undercoordination. Section 3 discusses the essentiality, criteria, and characteristics of the X:R-Y coupling systems including electron lone pair engagement, dipolar replacement, and charge polarity inversion.