Yājñavalkya Śikṣā 201

Where a word is shortened or lengthened, being slender-bellied, / that is to be known as euphonic substitution; ubhe suścandra is the illustration.

Now, the seven kinds of consonant-cluster masses:

the iron mass, the wooden mass, the woollen mass, the flame mass,

the clay mass, the wind mass, and the thunderbolt mass.