Bottom cloth radical - the indexing component. Pictures a hanging strip of fabric with a vertical sash. Anchors 幕 in the textile family and gives the word its concrete meaning: a large cloth (curtain, tent, theatre screen) hung up to cover or divide a space.
Top 莫 supplies the sound - mò shifted to mù, a vowel-only adjustment. Same phonetic series: 慕 (mù, yearn for), 墓 (mù, tomb), 暮 (mù, dusk), 模 (mó, model), 膜 (mó, membrane). Recognising any one of these makes the sound of 幕 immediate.
