Siman 180

One should not remove the tablecloth and the bread until after birkat hamazon. And that which is implied in Berakhot (42a) that they would remove the tables before birkat hamazon, this is because each one ate at a small table for himself, and they would remove the tables from before those reclining and not from before the one reciting the blessing. But we, who all eat at one table – one should not remove from it until after birkat hamazon (Tosafot and the Rosh ibid.). And it appears that for this reason there are places where they followed the custom of leaving the bread and the tablecloth before the one reciting the blessing, as the Tur wrote, see there, in order to act according to that which was in their time.

And the reason for the matter: because the blessing does not rest upon an empty thing. As we find with Elisha and the Shunammite, where he said to her: "What do you have in the house." And in Sanhedrin (92a): Anyone who does not leave bread on his table—never sees a sign of blessing. And so it is in Zohar Yitro (page 87b), and this is his wording: It is forbidden for a person to bless over an empty table, for the blessing from above does not rest in an empty place... And so it is in Zohar Terumah (4, 157b), see there. And our teacher, the Beit Yosef, wrote: But he should not bring a whole loaf and place it on the table. And if he did so, it appears like "those who set a table for Gad" mentioned by the prophet Isaiah (65:11). And specifically to bring it is forbidden, but if a whole loaf was previously lying there—he does not need to remove it (Taz and Magen Avraham). Therefore, on Shabbat, when the second challah of lechem mishneh is lying there—one should not remove it from the table. But pieces must necessarily be present, and if no pieces remained on the table—he should bring from another place. And in Job (20:21) it is written: "There is no remnant for his food, therefore his goodness shall not endure." From this our sages, of blessed memory, expounded to leave bread on the table, for from this itself it is implied that the remnant is pieces, as this is the meaning of "remnant."

And there are several reasons for the leaving of the bread: so that it will be ready for a poor person when he comes (Rashi). And furthermore: in order to give thanks to Hashem, may He be blessed, for His kindness and His goodness, that we have eaten and left over, as it is written: "eat and leave over." And likewise, so that it will be recognizable to all that we bless Hashem, may He be blessed, for preparing food for all His creatures. And furthermore: so that His blessing, may He be blessed, shall rest upon this table and upon this house, as we say in "May the Merciful One send us an abundant blessing in this house and upon this table that we have eaten upon." And for every flow of blessing, there must be something for the blessing to rest upon, as I have written. (Two reasons of these are in the Levush.)

In the days of the sages of the Talmud, when they would remove the small tables that were before the diners prior to mayim acharonim, and they would wash mayim acharonim in the place of the tables—they were required to sweep the floor of the house in the place of the tables prior to the washing, so that crumbs of bread would not remain there and become loathsome from the wash water. And even though it is permitted to destroy crumbs that do not contain a kezayit, nevertheless they were concerned lest the attendant be an am ha'aretz, as it is permitted to use an am ha'aretz as an attendant, and when he removes the bread from the table—he might also leave on the table crumbs that contain a kezayit, and afterwards they would fall from it when they remove the table from there, and they would become loathsome from the wash water. Therefore, one must sweep first. But for example with us, where we do not have removal of the table, and we wash the hands in a place where there is no concern of crumbs, for when we wash the hands at the table into a vessel we are careful that the water should not fall on the tablecloth, and consequently it will not reach the food. This is not the case with them, as they would wash with a vessel over the ground, and it is not the way to be meticulous that the water should not fall on the ground. Therefore, with us there is no need for sweeping the house first. And know that this which has been explained, that it is permitted to destroy crumbs that do not contain a kezayit, this is for example to throw them into water or fire. But it is forbidden to throw them in a place where people's feet tread, because it leads to poverty when one treads on a crumb of bread, as we say in Chullin (105b), see there. (See the Bach who is stringent even now to sweep for the reason that we wash before the table, see there. And according to what I have written there is no concern in this, and the custom is not so. And only the tablecloth, there are those who clean it slightly. Examine and you will find it simple.)

The early authorities wrote that it is the custom to cover the knife during birkat hamazon. On Shabbat and Yom Tov, it is not the custom to cover it. The reason is: because the table is similar to the altar, and regarding the altar it is written: "you shall not lift up iron upon them"; for iron shortens the days of man, and the altar lengthens the days of man, and it is not right that the shortener be lifted over the lengthener. Also, the table causes the lengthening of days through hospitality and atones for sin, and iron is the power of Esau. Therefore, on Shabbat and Yom Tov there is no Satan and no evil occurrence. And the custom of Israel is Torah. Furthermore, they stated a reason: because of an incident that occurred with a certain righteous man, who said "And build Jerusalem...", and was greatly pained over the destruction of the Beit HaMikdash, and he took the knife and killed himself (see Beit Yosef). Therefore, they decreed regarding this. They did not decree on Shabbat and Yom Tov, for when the incident occurred it was on a weekday, and they only decreed similar to the incident that occurred; as we say in Shabbat (60b) regarding the spiked sandal, see there. Furthermore: on Shabbat and Yom Tov it is forbidden to be pained, and it is impossible for such a mishap to occur. And know that now we are not accustomed to be careful in this, and we do not cover the knives during birkat hamazon.