Is it Permissible to Fold a Page as a Bookmark on Shabbat?

Answer: One of the forbidden labors on Shabbat is the labor of "writing." Anyone who writes on Shabbat, even if they write only two letters, is liable for violating Shabbat.

Making a mark by scratching wooden boards

In the Mishnah in Tractate Shabbat (103), our Tannaitic sages disagreed about whether one who scratches marks on wooden boards and the like is liable for "marking," or if the Torah only prohibited actual writing. According to Rabbi Yosi, one who makes a scratch on boards is liable for performing labor on Shabbat. However, the Tur (in section 340) ruled that the law does not follow Rabbi Yosi. Rabbi Ovadia of Bartenura also ruled this way in his commentary on the Mishnah. The Mishnah Berurah (in Biur Halacha s.v. "It is permitted") ruled similarly, as did the Shulchan Aruch and most early authorities.

However, it is rabbinically forbidden to make a mark on wood by scratching on Shabbat. From here, we can discuss whether a person who makes a mark by lightly scratching the paper of a book with their fingernail is at least rabbinically prohibited, like one who scratches wooden boards, or whether scratching paper is not considered marking at all and is therefore not forbidden on Shabbat?Scratching papers and parchments on Shabbat

Indeed, the Tur (section 340) wrote that it is permissible to mark a sign (such as a line) with a fingernail on a book on Shabbat, and the Shulchan Aruch (section 5) ruled similarly. Their reason for leniency is that scratching in a way that is not permanent, meaning it does not last for a long time, is not prohibited at all on Shabbat. This is because our Sages only prohibited scratching on Shabbat when the scratch is lasting, like one who scratches wooden boards as a sign, but the mark of a scratch in a book only remains for a short time, and therefore it is not included in the prohibition of marking on Shabbat. (However, it should be noted that it is forbidden to scratch the skin on Shabbat).

Some later authorities, including the Turei Zahav, wrote that one should only be lenient with books made of parchment, where the mark remains for a very short time, but with books made of paper, one should not be lenient. However, from the words of the Shulchan Aruch, it seems that there is no distinction in this matter, since in the time of the Shulchan Aruch, books printed on paper were already common, and nevertheless, he wrote to be lenient in this matter.

Therefore, it is permissible to make a mark like a line on the side of the page, because this mark is not permanent, as we have explained.

Rulings of Rabbi Ovadia Yosef courtesy of the "Daily Halacha" website