x1 has an hourglass form [shape] and is composed of material x2 with narrow region x3 and bulbs/wider regions x4
Also includes: shape with flanches, waistline, peanut shaped, dumbbell shaped, connected-teardrop shaped, the shape of p-orbitals, etc. This is a simple shape which has a narrow point with two bigger (and typically round) shapes on either end. Typically, two bulbs will be connected by the narrow region. The narrow region need not be a point. The bulbs need not be bounded; they will typically be roundish in some way (for example: cross-sectionally) but need not be so in general. The shape does need to be symmetric. Some simplicity in the abstract form, at least locally, is implied. See also: cnebo.