Despite their wide presence in hcp metals, incoherent tilt grain boundaries (ITGBs) are rarely studied.In this work, first-principles calculations were combined with atomic-resolution HAADF-STEM analyses to investigate the structure and formation of one special type ITGB in an Mg-Gd alloy featured with a Neckwear unique Gd-rich cluster segregation pattern.Our results suggested this ITGB as a mirror-glide boundary formed by gliding the atoms Female Contraceptives along [ $20ar{2}ar{1} $] of a { $10ar{1}4 $} coherent twin boundary due to its interaction with stacking faults.The ITGB can be further stabilized by partial-column Gd substitution, leading to the observed unique segregation pattern.