Every baby’s mouth has a labial frenulum – a thin band of tissue that attaches the upper lip to the gum. A lip tie happens when this frenulum is so thick or short that it limits the normal movement of the lip. In other words, the upper lip cannot flange outward as usual. By contrast, a normal frenulum (often just called labial frenulum) usually lets the lip move freely and often stretches or migrates up as a child grows. Lip ties are sometimes graded by attachment type (Kotlow classification): Class 1 (minimal restriction at the lip margin), up to Class 4 (frenulum tightly attached high on the gum). A Class 1 frenulum is typically not problematic, whereas a Class 3 or 4 can restrict lip mobility. However, attachment type alone doesn’t guarantee symptoms – it’s the functional restriction that matters. In fact, many infants with “low-set” frenula have no issues at all. Only when the tight frenulum prevents the lip from flanging (opening wide) do doctors label it a lip tie. It’s worth noting that a tight upper lip tie can contribute to spacing between the front teeth (a midline gap) and even make brushing the front teeth harder, since debris may get trapped under the lip. https://nutibes.com/lip-tie-in-babies/