text: In higher plants thylakoids are organized into a granum-stroma membrane assembly. A granum (plural grana) is a stack of thylakoid discs. Chloroplasts can have from 10 to 100 grana. Grana are connected by stroma thylakoids, also called intergranal thylakoids or lamellae. Grana thylakoids and stroma thylakoids can be distinguished by their different protein composition. Grana contribute to chloroplasts' large surface area to volume ratio. A recent electron tomography study of the thylakoid membranes has shown that the stroma lamellae are organized in wide sheets perpendicular to the grana stack axis and form multiple right-handed helical surfaces at the granal interface. Left-handed helical surfaces consolidate between the right-handed helices and sheets. This complex network of alternating helical membrane surfaces of different radii and pitch was shown to minimize the surface and bending energies of the membranes. This new model, the most extensive one generated to date, revealed that features from two, seemingly contradictory, older models coexist in the structure. Notably, similar arrangements of helical elements of alternating handedness, often referred to as "parking garage" structures, were proposed to be present in the endoplasmic reticulum and in ultradense nuclear matter. This structural organization may constitute a fundamental geometry for connecting between densely packed layers or sheets.