|M'sarteit - Tracing Lines|
The prohibition comes into force when the tracing is done for the purpose of guiding one’s hand in another, later act, such as writing, cutting, tearing, or sewing, as had been the case with leather hides in the Tabernacle’s construction.
The Biblical law covers only lines that will remain permanently, but Rabbinic law extends the prohibition to include impermanent lines.
It doesn’t matter whether a utensil is used (such as scratching a line with a knife on a plank of wood to cut it later to that measurement) or only one’s hand (as in creasing a piece of paper to be torn later).
The hardness or softness of the material used is irrelevant.
Practically, this would include tracing lines in modeling clay or dough (which in any event cannot be used on Shabbat).
Luckily, the prohibition does not include food. So one may scratch lines in a challah or on a cake’s surface (to ensure it is cut into equal portions, for example).
Also luckily for readers, creasing the corner of a page of a book to mark a place is permitted.
Adapted from the Shabbos Home by Rabbi Simcha Bunim Cohen