Siman 274
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א בּוֹצֵעַ עַל שְׁתֵּי כִּכָּרוֹת שְׁלֵמוֹת, שֶׁאוֹחֵז שְׁתֵּיהֶן בְּיָדוֹ וּבוֹצֵעַ הַתַּחְתּוֹנָה. הַגָּה: וְדַוְקָא בְּלֵילֵי שַׁבָּת (דִּבְרֵי עַצְמוֹ), אֲבָל בְּיוֹם הַשַּׁבָּת אוֹ בְּלֵילֵי יוֹם טוֹב בּוֹצֵעַ עַל הַעֶלְיוֹנָה (כָּל בּוֹ וְהַגָּהוֹת מַיְמוֹנִי פ"ח מֵהִלְכוֹת חָמֵץ וּמַצָה), וְהַטַּעַם הוּא עַל דֶּרֶךְ הַקַּבָּלָה.
1 The Laws Pertaining to Breaking Bread on Shabbos, 4 Seifim: Every person is obligated to break bread on two [whole] loaves. One holds them both in his hands and breaks the bottom one. Rema: Specifically on the night of Shabbat. However on Shabbat day, or on the night of Yom Tov, we break the top one [Kol Bo; Hagahos Maimoni] and the reason is according to the Kabbalah.
ב מִצְוָה לִבְצֹעַ בְּשַׁבָּת פְּרוּסָה גְּדוֹלָה שֶׁתַּסְפִּיק לוֹ לְכָל הַסְעֻדָּה וְעַיֵּן לְעֵיל סי' קס"ז.
2 It is a mitzvah on Shabbat to break a big piece that will last you through the whole meal [and see above Siman 167].
ג אֵין הַמְּסֻבִּין רַשָּׁאִים לִטְעֹם עַד שֶׁיִּטְעֹם הַבּוֹצֵעַ, וְאִם יֵשׁ לִפְנֵי כָּל אֶחָד לֶחֶם מִשְׁנֶה יְכוֹלִים לִטְעֹם אַף עַל פִּי שֶׁעֲדַיִן לֹא טָעַם הוּא.
3 Those participating in the meal may not partake [of the bread] until the person breaking bread [first] partakes [of the bread that he cuts]. If, however, two loaves are placed before each one of them, they may partake of their bread even though he has not yet partaken of his.
ד סְעֻדָּה זוֹ וְשֶׁל שַׁחֲרִית אִי אֶפְשָׁר לַעֲשׂוֹתָהּ בְּלֹא פַּת.
4 This meal, and the meal of the daytime, cannot be done without bread.