This article aims to provide a unified explanation of the systematic dual behavior of glide formation in Korean. I argue that the optionality or obligatoriness of glide formation is driven by the emergence of unmarked syllable structure in the sense of McCarty and Prince(1994). It is assumed that faithfulness constraints on segment dominate relevant syllable structure constraints which in turn dominate another faithfulness constraint on mora. Generally, the segment faithulness is obeyed at the expense of violating syllable structure constraints. But those latter constraints are obeyed in one condition where the segment faithfulness is not at issue. Just in that case, the unmarked syllable structure is emergent. I review the previous treatments in Han(1990) and Lee(1996), overcoming their limitations while preserving their insights. In a nutshell, the central advantage of a solution of dual aspects of glide formation along the lines in this article is that it enables us to maintain a single ranking of constraints, dispensing with any additional stipulations, which is a basic tenet of standard OT.