1) Tefillin may be purchased only from an expert, Godfearing scribe who is proficient in the proper spelling of all the words in the Biblical sections written in the tefillin. If, be’di’avad (after the fact), one purchased tefillin from a scribe who does not meet these qualifications, then he must have the tefillin checked to ensure that the words are spelled correctly and that the letters are drawn in accordance with halachah.[224]

2) Tefillin that may be presumed to have been written properly never require checking. Nevertheless, it is preferable to have them checked on occasion to ensure that they are valid for use. [225]

a. Particularly pious Jews make a point of having their tefillin checked each year during the month of Elul, though this is not required according to the strict halachah. [226]

b. Tefillin that are not worn regularly should be checked twice every seven years — meaning, once every 3.5 years.[227]

3) If a person purchased tefillin on the assumption that they were valid for use, and several years later discovered that they were, in fact, not valid, he should feel distressed over having failed to perform this mitzvah throughout this period. He should increase his involvement in Torah learning, and, if he had not been previously accustomed to wearing tefillin Rabbenu Tam, he should commit to wear tefillin Rabbenu Tam each day (in addition to standard tefillin).[228]