Round balloons have several features in common that give them their structure.
The body of a round balloon is the portion which is designed to be inflated. It is oblong or spherical in a bulb molded balloon, and may be X- or star-shaped in a balloon manufactured on a fluted mold.
Typically, a balloon reaches its rated size when its body is inflated to a high enough pressure that the balloon begins to store additional air in its neck instead.
The neck of a round balloon is the portion which connects its body to its inflation outlet. It is also the portion of a balloon which is tied into a knot to keep it inflated.
A balloon's neck is always cylindrical in shape, but its length and width may vary between manufacturers. Some manufacturers of balloons with longer necks advertise them as being "easier to tie." Making balloons with shorter necks, on the other hand, may be more cost-effective.
The neck of a balloon is not designed to be inflated. But if a round balloon is inflated past its rated size, the neck will typically begin to inflate when the body's pressure becomes high enough.
Not all round balloons' necks may be inflated completely, however: if the ratio of body to neck diameter is too high, or the body-neck transition is too severe, the pressure required to completely inflate the balloon's neck is higher than the burst pressure of the body. Fluted balloons are especially difficult to completely inflate down their necks.
The lip (or bead) of a balloon surrounds the inflation outlet, making it easier to grasp the balloon for inflation. Having a reinforced lip also keeps the inflation outlet open rather than collapsed on itself, making inflation by mouth much easier.
Lips are usually created during the manufacturing process by small rotating brushes which roll latex from the balloon's unfinished neck. Since this typically happens before the balloon is vulcanized, the roll effectively glues itself together and it is very difficult, though not impossible, to unroll a finished balloon's lip.
When balloon molds are dipped in latex, they are dipped neck up. The latex which has collected on the form then settles, and as the balloon is removed from the latex bath the excess drips off the bottom "point" of the mold. Typically, this results in a circular, eraser-sized thickening at the top end of the balloon, called its drip point.
The latex of a balloon's drip point can, in some instances, be thick enough to be pierced without popping the balloon. The "needle-through-balloon" magic trick typically involves piercing a balloon through its uninflated neck and its drip point.
Manufacturers' dipping procedures vary, so some balloons have fainter or differently-shaped drip points.
The point at which the neck of a balloon meets its body is called its body-neck transition, the place where if you ran a finger down the balloon from body to neck, your finger "turns." This is chiefly a feature of round ballons; paddle-shaped and twisting balloons do not have such a transition.
The three balloons at right are arranged in decreasing order of the sharpness of this transition. The transition on a round balloon is a focal point of negative curvature — and high stress — that increases with the transition's sharpness. In order to inflate a balloon down its neck, this high stress must first be overcome, either by high pressure in the body or by pre-stretching. Thus, balloons with gentle transitions are more likely to inflate completely down their neck to their lip without bursting.