The knot that is shaped like a yud on the tefillin shel yad (the tefillin we bind on our arm) should always be touching the bayit (the box of the tefillin). According to some poskim this applies even when the tefillin are not in use.
There are opinions that say that it’s sufficient if the yud touches the titura (the base), even if it does not touch the bayit itself. Certainly, it’s sufficient if part of the yud touches the bayit.
We also have to make sure that all the retzuot (straps) of the tefillin, including the yud and the area of the kesher (knot) always remain black. However, it’s acceptable if they give a black impression. Some opinions are lenient even if they are mostly black. The part of the retzuah that goes into the kesher and is not outwardly visible need not be black. In addition, it’s important to make sure that the entire yud is the right size: at least the beginning of the yud where it protrudes from the knot should be a minimum of nine millimeters wide.
Rav Yosef Yeshaya Braun, shlita, member of the Badatz of Crown Heights, One Minute Halachah #322, reprinted from crownheightsconnect.com
