The Torah obligation to return lost property is grounded in Devarim 22:3, which commands returning every lost item of one's brother — and the Gemara in Bava Metzia 27b derives from the garment singled out in those verses that any item bearing distinguishing marks and having claimants triggers the obligation to proclaim the find.
The Torah-level status of simanim as the basis for returning a lost object is established by Rava in Bava Metzia 28a, who reads the word 'derosh' in Devarim 22:2 as directing the finder to scrutinize whether the claimant is a swindler — which can only be done by checking distinguishing marks, proving that simanim operate at the level of Torah law.
The Gemara in Bava Metzia 23b resolves that weight serves as a distinguishing mark, and from this it derives that measure and number equally qualify as simanim — a ruling codified by the Rambam (Mishneh Torah, Robbery and Lost Property 13:1–6), who lists measure, weight, number, and the location where the item was lost as prime examples of muvhak (distinctive) simanim, and echoed by Shulchan Arukh, Choshen Mishpat 267:7–8, which states the same four categories explicitly.
A critical procedural distinction governs how simanim are used in the return process: Bava Metzia 28b rules that stating the type of lost item without providing its distinguishing marks is insufficient to compel return, and that even a known swindler cannot receive the object even if he correctly names the simanim — while Shulchan Arukh, Choshen Mishpat 267:1–4 specifies that the finder announces the category of item (garment, animal, documents) and the claimant must supply its muvhak simanim before it is returned.
When two claimants each offer a different type of siman, the Gemara in Bava Metzia 28a rules that competing simanim of equal strength require the item to be held, while testimony from witnesses overrides simanim — and Shulchan Arukh, Choshen Mishpat 267:12–15 elaborates that one who gives the length-measurement prevails over one who gives only the width, since width can be approximated by how an owner drapes the garment, whereas one who gives both length and width is overridden by one who correctly states the weight, since weighing a garment is unusual and therefore constitutes a stronger, more distinctive siman.
A siman that is prone to being trampled — such as a distinguishing mark on bundles of grain left in a public area — is the subject of a machlokes between Rabba and Rava in Bava Metzia 22b: Rabba holds it does not qualify as a siman because the owner anticipates its destruction and despairs, while Rava holds it does qualify and the finder must still proclaim the find — and the practical instruction in Arukh HaShulchan, Choshen Mishpat 267:1–20 confirms that once any proper siman is given, we do not fear that a swindler who happens to know the marks will fraudulently collect the item.
The Rambam in Mishneh Torah, Robbery and Lost Property 14:1–5 further extends the siman framework to accessories: the donkey is singled out in the Torah to teach that a siman on its saddle-cushion, though secondary to the animal itself, suffices to return the animal — establishing that simanim need not be on the object's body proper.
The distinction between a siman muvhak and a weak siman is illustrated in Bava Metzia 18 by Rav Ashi, who rules that a hole in a document next to a specific letter is a clear-cut distinguishing mark and permits return, whereas a hole mentioned without its precise location is not considered a muvhak siman — and Rav Ashi himself notes uncertainty as to whether simanim operate at Torah or rabbinic level, a question Rava had already resolved in Bava Metzia 28a.
Igros Moshe (CM 2:45) rules that when someone finds a lost object, they should record all identifying marks and distinctive features; if the owner is unknown, the finder must preserve testimony from three knowledgeable witnesses documenting those simanim, after which the finder may use the object, with the matter to be clarified should the owner later appear and provide the marks.