Śaunaka Śikṣā 16

16

e ai tu kaṇṭhatālavyau o au kaṇṭhoṣṭhajau smṛtau
sparśānāṃ karaṇaṃ spṛṣṭaṃ antasthāsvīṣaducyate

'e' and 'ai' are remembered as palato-guttural; 'o' and 'au' as labio-guttural. / The articulation of the stops is closed (spṛṣṭa); in the semivowels, it is said to be slightly [closed] (īṣat).