In a ring structure, an unlabeled atom at the angle where two lines join is assumed to be a carbon atom. 2. unlabeled atoms joined to carbon atoms, which are not directly part of a ring structure, are assumed to be oxygen atoms.

Ring structure is defined as the structure in which one or more series of compounds are joined or linked to form a ring.  Ring can exist in different sizes such as cyclopropane (small ring: three carbon atoms) and cyclohexane (large ring: six carbon atoms).

In ring structure, there is only carbon atoms linked with hydrogen atoms which are identified by the linkage of two lines.

See the attached figure: cyclopropane (unlabelled atoms are carbon atoms with hydogens).

Thus, in ring structure, an unlabelled atom at an angle where two lines joined assumed to be carbon atom.