The Tefillin boxes that we wear contain parchment upon which are written four brief sections of the Torah: the section of "Kadesh Li" (Shemot 13:1-10); the section of "Ve'haya Ki Yevi'acha" (Shemot 13:11-16); the section of "Shema Yisrael" (Devarim 5:4-9); and the section of "Ve'haya Im Shamo'a" (Devarim 11:13-21). In the Tefillin Shel Rosh, each section is written on a separate parchment, which is then placed in one of the four compartments of the Tefillin Shel Rosh. The Tefillin Shel Yad, by contrast, contains just a single, long piece of parchment upon which all four sections are written successively.
The Shulchan Aruch (in Siman 32) discusses the proper sequence when writing or arranging the parchments of the Tefillin. Halacha requires that the four sections be arranged in the sequence in which they appear in the Torah, as we listed them above. With regard to the Tefillin Shel Yad, this means that the scribe must write the parchment in this sequence. With respect to the Tefillin Shel Rosh, this means that the four parchments must be arranged in this sequence when they are inserted into the compartments of the Tefillin Shel Rosh.
Regardless of how beautiful a pair Tefillin may look externally, and even if it satisfies all the other requirements of Tefillin to the strictest standard, it is disqualified for use, if the parchments are not written in the proper sequence; meaning one does not fulfill his obligation at all with such Tefillin if written out of order. Therefore, it behooves a person to ensure that he purchases Tefillin only from a reliable, reputable, God-fearing Sofer (scribe) or retailer. A person who receives a pair of Tefillin obviously cannot determine whether or not the parchments were written correctly, and must therefore ascertain the credentials and integrity of the Sofer or retailer when purchasing Tefillin, for otherwise he runs the risk of going many years without, Heaven forbid, fulfilling this very important Mitzva.
