Double Seal
10. For these halachos, when we mention the concept of a “seal within a seal,” it does not actually require the form of one seal inside another. It just means there must be two signs nonkosher meat wasn’t swapped with kosher meat. There is a machlokes haposkim about the nature and purpose of the seal, with multiple practical ramifications, as will be explained (עמק התשובה ח''ב סי' ע', גם מתוך קו' 'חתימה טובה' שטרם בא לדפוס ע''י ש''ב הג''ר חנוך בלייער בעמ''ס חלב כהלכתה).
11. Sign. Some hold a seal is essentially a unique indication as a sign – a double seal is two signs – that this is indeed the item that was sent and it was not swapped (רמב''ם פי''ג מאכ''א ה''ח).
12. Effort. Others hold the value of a double sign is in the effort it entails: while it is not impossible, one will not go to the great effort of forging two seals (רש''י ע''ז דף ל''א ע''א ד''ה ומיהדק, דף ל''ט ע''ב ד''ה חילחית, שו''ת עבודת הגרשוני סי' צ''ח, מהרי''ל סי' ל''ח ).
13. The Shulchan Aruch’s wording includes both options. In one place, he uses the Rambam’s wording, that a seal is a sign (סי' קי''ח ס''ד). Regarding a knot, he writes that it must be an unusual knot, and it is problematic for there to be no sign. His words imply that one seal can serve as a sign and one seal can serve to increase the effort necessary to forge it.
Types of Seals
14. Two letters. Back when not everyone recognized the shape of Jewish letters, it was enough to write two Jewish letters, e.g., א and ב, where the item’s package closed in such a way that it would be clear if the package was opened (רמ''א יו''ד סי' ק''ל ס''א). We are not concerned the non-Jew would forge new letters, as it would be difficult for him to forge two letters, and he would not go to such lengths. Two letters were considered two seals, and one letter was considered one seal, which helped in cases where one seal is enough (שו''ע יו''ד סי' קי''ח ס''ג).
15. Nowadays, everyone knows the Jewish script and it is easy to find the shape of Jewish letters on a computer or the like, so it is easy to duplicate them. Thus, some say letters alone are not considered seals (י''א בשו''ע שם). Especially today, in the age of printing, printed Jewish letters alone are not a proper seal. One may certainly not rely on this in Eretz Yisroel, where even non-Jews and irreligious Jews recognize the letters of Lashon HaKodesh (ארחות חיים יין נסך סי' כ''ג אות כ''ט, פרי תואר סק''ו).
16. Stamp. A stamp, e.g., of a kashrus body, is only considered one seal even though it has multiple letters since the seal is created in a single action of pressing down the form of the stamp.
17. Box nailed shut. Earlier poskim discussed whether a cover nailed on top of a box containing meat or wine is considered a seal. Some consider it a single seal even though there is nothing unique about it since it takes effort to open and close it again afterward (ערהו''ש סי' קי''ח, שו''ת אבני צדק יו''ד סי' נ', שו''ת שמש צדקה יו''ד סי' ט''ו ). This fits with the opinion that a seal causes more effort (12).
18. Corked wine bottle. The poskim also discuss whether a cork is considered a seal when sending wine in a bottle. On the one hand it takes effort to remove the cork with a corkscrew, but on the other hand, a cork is something normally in a bottle, so perhaps it is not considered a seal. In practice, the poskim hold it is considered a single seal.
19. Single forgery-proof seal. The poskim also discuss the status of a forgery-proof seal. Do we say two seals are necessary for two signs and this is only one (כדעת הרמב''ם ), or perhaps the whole point of two signs is to ensure no swap happened, so if a single seal makes it impossible to swap (as will be explained below), it has the benefit of not being forgeable (שו''ת אג''מ יו''ד ח''א סי' נ''ו). In practice, many poskim follow this second approach.
20. Key. There is a machlokes haposkim whether kosher food in a non-Jew’s hands that was closed up with a key held by a Jew has the status of a seal. Some say where two seals are required, a key plus a seal is considered two seals, but where one seal is enough, a key alone is not considered a seal (שו''ע סי' קי''ח ס''ד).
21. Some hold the Rama’s opinion is that for something in the hands of a non-Jew that suffices with one seal, a key alone does not help even b’dieved, as it can easily be opened with something else, and the non-Jew will go to the effort to fake it (ש''ך סקכ''ב, לבוש, מנחת יעקב כלל ל''ב סל''ז).
22. However, others hold the Rama’s opinion is that just as one can rely on a single seal b’dieved (above, 4), one may similarly be meikel b’dieved with a key alone (ט''ז שם סק''ג, סק''ו, ב''ח, פרי חדש סקח''י). This is the consensus of most Acharonim (שו''ת חת''ס יו''ד סיק קכ''ו); one can certainly rely on the meikel opinion in a pressing situation or a case of potential substantial monetary loss (ערוה''ש סקט''ז, חכ''א כלל ע' ס''ד ).
23. Keys today. This entire discussion is about olden-day keys. Back then, one could find a different key that would work without much effort, and it was also easy to copy a key. Today, though, we use good locks and it is almost impossible to find an identical key to open them, and it is impossible to copy a key without going to some lengths. Thus, the poskim agree one can l’chatchila rely on something closed with a key even according to the Mechaber. One can certainly rely l’chatchila on a high-quality lock even in a situation requiring a double seal (כף החיים סי' קי''ח סקמ''ה, מזמור לתודה, ספר יין מלכות פי''ב סקי''ב אות א').
