横 (héng) means 'horizontal' — distinguishes the side-blown flute (held across the body, like Western flutes and the Chinese
笛子 dízi) from the end-blown
竖笛 (shùdí, 'vertical flute', e.g. recorder or
箫 xiāo). The image
牧童横笛 ('shepherd boy with horizontal flute') is a classic motif in Tang poetry and ink painting.
