While it is true that some antiparallel beta sheets have neighboring strands that are adjacent in primary sequence, other cases do exist. As for parallel beta sheets, the polypeptide chain between strands could be connected via a loop or a helix if the two strands are close in primary sequence.
The mapping of strands on the primary structure has been studied thoroughly, and there are two types of diagrams to quickly show what is going on. For the "TOPS" diagram, we pretend to look down the strands, and up and down triangles represent strands in one or the other direction:
Circles represent helices. If you look closely at the connecting lines, you can see whether the connection is above or below the plane of the paper.
The other representation has the sheets in the plane of the paper, e.g.:
- Most sheets are mixed (combo of parallel and antiparallel).
- Most neighboring strands are not adjacent in sequence.
- When two neighboring parallel strands are adjacent in sequence, there is a loop or a helix between them (or a strand from a different sheet) to connect them in the proper orientation.