tongue height according to the vertical position of the tongue high vowels, also referred to as close; low vowels, also referred to as open; intermediate- close-mid and open-mid
frontness vs. backness of the tongue - according to the horizontal position of the highest part of tongue.
lip rounding - whether the lips are rounded o-shape or spread no rounding when the sound is being made.
tenseness of the articulators - refers to the amount of muscular tension around the mouth when creating vowel sounds. Tense and lax are used to describe muscular tension.