The folds, called gyri in the cerebrum and folia in the cerebellum, with grooves called sulci between the gyri and folia, vary considerably from person to person and even between the right and left hemispheres in the same person. The human brain has only an approximate symmetry. Notice the considerable differences between the brains of these two individuals and right–left differences even in the same individual.

Nevertheless, there is enough consistency for individual gyri and sulci to have anatomical names—precentral gyrus, postcentral gyrus, central sulcus, parieto-occipital sulcus, etc.—and for neurosurgeons to know their way around by familiar landmarks when they’re in there.

Update

In the comments, one reader asked why the folds are called gyri in the cerebrum but folia in the cerebellum—why two words for what seems like the same thing? I think the answer may be of enough general interest to answer here in the main post.

It does seem illogical to have two words where one would do, but it’s the official international terminology (Terminologia Anatomica, Gray’s Anatomy, etc.), so like it or not, we’re stuck with it. It’s not as illogical as it might seem, though.

The history behind the separate terms is unclear. Unlike many of today’s other familiar anatomical terms, these weren’t used by the Greeks or Romans. Gyrus apparently was first used in the 16th century, but it’s unknown who coined it. One can see the reasoning, though. Gyrus comes from the Greek for “circle,” but over the history of language, came to connote “twist” or “turn” (as in gyrate, gyroscope). You can see in the photos above that the word aptly describes the twisting folds of the cerebral hemispheres, as they turn every which way.

Folium (plural, folia) is Latin for “leaf,” as in the word foliage. It then came to denote the pages (leaves) of a book, or a page size (as in folio books), as well as other flat, thin things (as when one “exfoliates” the skin to remove surface cells). If you look at the cerebellum drawing below, you see its folds are thin and parallel, more like the pages of a book than the twisting, turning “gyrations” of the cerebral cortex.

Hence the difference in words.

Photo sources

Cerebrum, top left: Brain Png - Human Brain Superior View, Transparent Png , Transparent Png Image - PNGitem

Cerebrum, top right: https://quizlet.com/289753532/human-brain-superior-view-diagram/

Cerebrum, bottom: https://www.mheducation.com/highered/product/anatomy-physiology-unity-form-function-saladin/M1259277720.html

Cerebellum: Credit on image.