It is the custom to make it twenty-two lines, and these are the beginnings of the lines: "Shema," the Name of Hashem after "V'ahavta," "et hadevarim," "levanekha," "u-veshokhbekha," "bein," "ve-hayah," "mitzvah," "bekhol," "yoreh," "esev," "pen," "ve-hishtachavitem," "ha-shamayim," "va-avadtem," "ve-samtem," "otam," "otam," "ba-derekh," "u-vish'arekha," "asher," "al ha-aretz." The first "letotafot" is missing a letter, the second "letotafot" is full with a vav. "Le-ot al yadekha," even though "yadekha" is a plural expression—nevertheless it is missing a yod after the dalet. The first "mezuzot" is with one vav, the second "mezuzot" is with two vavs. "Tiroshkha" is missing a vav after the resh. "Ha-tovah" is missing a vav after the tet. And so too "la-avotekhem" is missing a vav after the beit (Tur). All of this is by tradition reaching back to Moses from Sinai. One must be careful with the full and missing spellings as with tefillin and a Sefer Torah. If one filled a missing letter or omitted a full letter—it is invalid. (And in the Shach, subsection 8, there is an error in the word "levavekha," and it should say "levanekha.")
ונהגו לעשותה עשרים ושתים שיטין, ואלו הן התחלות השורות: "שמע", הוי"ה של אחר "ואהבת", "את הדברים", "לבניך", "ובשכבך", "בין", "והיה", "מצוה", "בכל", "יורה", "עשב", "פן", "והשתחויתם", "השמים", "ואבדתם", "ושמתם", "אותם", "אותם", "בדרך", "ובשעריך", "אשר", "על הארץ". "לטטפת" הראשון חסר, "לטוטפת" השני מלא וי"ו. "לאות על ידך", אף ש"ידיך" הוא לשון רבים – מכל מקום חסר, יו"ד אחר הדל"ת. "מזוזת" הראשון בחד וי"ו, "מזוזות" השני בשני ווי"ן. "תירשך" חסר וי"ו אחר הרי"ש. "הטבה" חסר וי"ו אחר הטי"ת. וכן "לאבתיכם" חסר וי"ו אחר הבי"ת (טור). וכל זה בקבלה עד למשה מסיני. ויזהר במלא וחסר כבתפילין וספר תורה. ואם מילא החסר או החסיר המלא – פסול. (ובש"ך סעיף קטן ח יש טעות בתיבת "לבבך", וצריך לומר "לבניך".)