What are level tones in Hmong-Mien languages?

In Hmong-Mien languages, level tones are tones spoken at a relatively steady-state pitch, as opposed to contour tones, which involve pitch movements like rises or falls.12

These languages are known for having large and complex inventories of tones, and level tones are a significant component of these systems. For instance, the Shidongkou Hmu ("Black Miao") language features five distinct level tones spoken at different pitches, which represents a world-record number of pitch level distinctions. While some Hmong-Mien languages utilize both level and contour tones to create complex tonal contrasts—such as the 12 tonal contrasts found in Longmo Bunu and Zongdi Hmong—the level tones specifically function by maintaining a consistent pitch to help distinguish words that are otherwise identical in their sequence of consonants and vowels.12