Gemara

R. Meir, R. Yehudah, Hillel and R. Yehoshua ben Korchah used to expound Lashon Hedyot (the text that people used to write in documents, even though Chachamim did not enact it).

(Beraisa - R. Meir): The sharecropper accepted in the contract 'if I will leave it fallow and not work it, I will pay what it would have produced'.

Rishonim

Rosh (Bava Metzia 9:3,4): R. Meir used to expound Lashon Hedyot as if it was a text instituted by Chachamim. E.g. even if one did not write 'your daughters (from me will be fed until they marry)' or 'your sons (will inherit your Kesuvah)', it is as if he wrote it, for it is a Tenai Beis Din. Also Lashon Hedyot, since people normally write it, is as if it was written. We cannot say that it is in force only when it was written. If so, this is obvious. It is not Asmachta!

Rambam (Hilchos Ishus 16:7): Ge'onim in all the Yeshivos enacted that a widow collects from Metaltelim, just like they enacted for creditors. This enactment spread throughout most of Yisrael.

Rambam (8): In every place we know of, the custom is to write in the Kesuvah 'from land and Metaltelim.' This is a great enactment that great Chachamim instituted. This is a monetary Tenai, so a widow collects due to the Tenai, not from the Ge'onim's enactment.

Rambam (9): If one did not write this in the Kesuvah, if he knew about the Ge'onim's enactment, she collects (from Metaltelim). If we are unsure if he knew, we must deliberate greatly. We cannot make orphans pay based on an enactment of Ge'onim that was not written (in the Kesuvah), the way we rule based on Tenai Kesuvah enacted by the Great Sanhedrin.

Magid Mishneh: It is not like Tenai Kesuvah because this enactment was made after the Gemara (was completed).

Rambam (Hilchos Malveh 11:11): All the latter Ge'onim enacted that a creditor can collect Metaltelim of heirs. Yisraelim in the entire world rule like this. However, in the west they write in loan documents that he can collect 'from land and Metaltelim, in his lifetime and after death', so he collects due to his Tenai in addition to the enactment. This is a great fence, lest the borrower not know about the enactment and orphans' money be taken improperly. The enactment of latter Chachamim cannot obligate orphans.

Rivash (392, cited in Kesef Mishneh and Mishneh l'Melech): The Rambam agrees that we collect even if it is not written. Writing avoids the need to rely on the enactment, and avoids the problem of one who did not know about the enactment and did not intend to put a lien on his Metaltelim. Also, writing enables to collect from Metaltelim even if the orphans have land. The enactment was only for when the orphans have no land, so the lender will not lose. In Hilchos Ishus (16:9), the Rambam says that if one did not write a lien on Metaltelim in the Kesuvah, it is not as if he wrote it. This is where it is the custom to write it. Since he did not write it, he put a lien on his property only according to Gemara law. Alternatively, in the days of the Rambam the enactment regarding Kesuvah spread through most of Yisrael, not all of Yisrael, like the enactment for loans spread. Nowadays everyone knows that we collect even from Metaltelim of orphans; not everyone knows the law according to the Gemara.

Beis Shmuel (EH 100:2): Now that the enactment regarding Kesuvah spread throughout Yisrael, even if he omitted it, it is as if he wrote it.

Rosh (Kesuvos 6:5): R. Tam says that the Kesuvah is collected from Metaltelim because men write in the Kesuvah a lien on Metaltelim Agav (along with) land.

Korban Nesan'el (70): Tosfos cites R. Tam to say that the enactment that creditors have a lien on Metaltelim applies also to Kesuvah. Nevertheless, we write in the Kesuvah, lest a man not know, and she collect improperly.

Poskim

Shulchan Aruch (CM 42:15): We follow a text that is normally written in documents, even if it was not enacted by Chachamim, rather, by commoners in a certain place. Even if it was not written, we consider it as if it was written.

Rema: The same applies to enactments of the Tzibur and customs of the city.

Beis Yosef (DH Kol): Tosfos, the Rosh, the Rashba, Rav Hai Gaon, R. Chananel, the Nimukei Yosef, the Ran and (it seems) Rashi all hold that something normally written is as if it was written, even if it was not written. We rule based on a custom followed in most places. If it is only in some places, we follow it only in those places. Ha'Rav Bartzeloni (brought in Nimukei Yosef 61a DH Garsinan) explained differently (expounding Lashon Hedyot means like we expound Pesukim, e.g. if it is not needed to teach about this, it teaches another matter). The Rashba rejected this.

Question (Beis Yosef, ibid.): The Rambam (Hilchos Ishus 16:9) says that if a man did not write a lien on Metaltelim in the Kesuvah, and we are unsure whether he knew about the enactment, we must deliberate greatly. It seems that he does not hold that it is as if it is written. Some say (to explain the Rambam) that if commoners started the custom themselves, it is as if it was written, but not if Acharonim initiated it. However, the Rashba says that a consensus of the Tzibur is stronger than a custom of commoners! Perhaps a consensus of the Tzibur is stronger than an enactment of Ge'onim, for everyone knows about the consensus. This is difficult.

Answer #1 (Beis Yosef): The Rambam discussed only places, unknown to us, where it is not normally written in the Kesuvah. Alternatively, he discusses places where the custom did not spread through the entire country. We find that the Rambam follows customs of the country, e.g. the dowry written is a third or a fifth more than the true amount (Hilchos Ishus 23:11,12)

Answer #2 (Shach 36): It is as if was written because we assume that he intended to write it. The Rambam was unsure only about one who did not know about the custom. He discusses only when there is a custom to write, for then one can say 'I did not know about the enactment. I saw women collect from Metaltelim, but I thought that it was because the man wrote a lien. This is why I did not write a lien!' However, if there is no custom to write it, surely he saw women collect from Metaltelim, and he should have realized that there was an enactment. The Rivash supports this. Also the Rashba rules that we expound Lashon Hedyot, but it is according to the judges' discretion: one cannot say that a woman knows the text normally written in documents, and assume that she commanded the scribe accordingly!

Shulchan Aruch (71:14): If a document has Ne'emanos (to believe the lender) and it is the custom in the area that many write this without asking the borrower, if the borrower denies authorizing it and the witnesses say that they wrote it only because it is written in other documents, we do not rely on it. This is because it is not a clear custom to write this in all documents.

Question (Beis Yosef DH u'Mah she'Chosav v'Im): Here, the Tur brings only the opinion of Sefer ha'Terumos, that we do not rely on Ne'emanos if the witnesses say that it was written not because the borrower said to do so, rather, because scribes normally write this in all documents. In Siman 61 he brought only the Rosh's opinion, that anything people in a country normally write, such as Ne'emanos or '(he commanded to) write it in public', can be written without a command. One who authorizes a document intends for the local custom!

Answers #1,2 (Beis Yosef): Perhaps Sefer ha'Terumos discusses when it is not the custom to write this in all documents, and the Rosh discusses when it is written in all documents. Alternatively, the Rosh discusses when there are no witnesses saying that the borrower did not authorize it.

Rebuttal (Shach 33): The second answer is inadequate. If one who authorizes a document wants it to be like the local custom, it does not matter if witnesses say that he did not specify! The Beis Yosef retracted from this. Therefore, in the Shulchan Aruch he relied on the first answer, and wrote 'many write this without asking the borrower.' But this is difficult, for the Tur and Sefer ha'Terumos explicitly say that it is written in all documents!

Answers 3,4 (Shach): Bedek ha'Bayis says that Sefer ha'Terumos discusses when in all documents it is written just for beauty of the document, not so that there should be Ne'emanos without authorization. I say that the Rosh discusses when the borrower knew the custom, and Sefer ha'Terumos discusses when he did not.

Beis Yosef (YD 174 DH Mochar, citing Rashba 3:435): If a group stipulated that whoever says 'I sell l'Shinsh' can redeem it at any time, even without any Tenai (at the time of the sale), it is like a loan. Since they stipulated from the beginning, it is as if they stipulated later at the time of the sale. We find that Tana'im expounded Lashon Hedyot, even though it was not a proper Tenai. Since it was the custom, it is as if they stipulated.