Rules of positioning the tefillin 1) Rule #1: The tefillin ‘shel yad’ must be placed on the “weak” arm. For most people, that is the left, but for many (yours truly included) it is the right. (Note: Below we will learn just how to classify a person who is dexterous in both hands, i.e. classic ambidextrous or otherwise.)
2) Rule #2: The ‘bayis’ of the ‘shel yad’ should not be placed too high or too low on the upper arm. Here’s the formula: If one had an X-ray machine, he could see his entire upper-arm bone (I’m not sure of its clinical name), which extends upward to his shoulder. The proper place of the ‘shel yad’ – according to most opinions – is somewhere on the lower half of that bone, but not too close to the elbow. Practically, this means that the entire ‘bayis’ should be resting on the curvature of the biceps muscle. When one flexes his arm at the elbow, he should be able to fit two fingers comfortably between his elbow and the base of the ‘bayis’. If the ‘bayis’ looks like it is hanging over the curvature on both sides, he may want to consider getting smaller tefillin! (Note: There is a minority opinion which permits the box to extend above the mid-point. This opinion may be relied upon when there is no other option. There is no opinion which allows the ‘bayis’ to be too close to the elbow!)
3) Rule #3: When the ‘bayis’ is tightened around the arm, it should be angled down slightly toward his heart, not upward toward the ceiling.
(The Shulchan Aruch briefly discusses the proper placement of the ‘shel yad’ for one whose one hand or arm is amputated, r”l. We will not go into that detail presently.)
4) Rule #4: The knot which forms the letter ‘yud’ that is close to the ‘bayis’ of the ‘shel yad’ should actually touch the bayis – always! Many tefillin-sellers will suggest or affix a piece of sinew-string which basically ties the knot to the ‘bayis’. (Ed: I have that, but I am “knot” in”sinew”-ating that you must!)
5) Rule #5: The ‘bayis’ should be placed on the arm in such a way that the ‘yud’-knot is on the inside of the ‘bayis’, and the wider part of the base, where the ‘retzua’ passes through (called ‘ma’avarta’ - passageway), is at the top, closer to the shoulder. You will note that these conditions make it necessary for “righty” and a “lefty” to obtain different kinds of ‘shel yad’. (Note: If a lefty had to borrow the tefillin of a righty, in a pinch, or vice versa, that may be done. In that case, it is preferable that the ‘bayis’ be turned around 180-degrees and positioned “upside-down”. In that way, the knot will still face inward, even though the ‘ma’avarta’ will be at the bottom rather than at the top.)
6) Rule #6: No separating barriers (Heb: ‘chatzitza’) should come between the tefillin and the body – especially the ‘bayis’ and the part of the ‘retzua’ that holds the ‘bayis’ down to the arm. This applies to the ‘shel rosh’ as well. For example, a person should make sure that his sleeve is not stuck under the ‘bayis’ of the ‘shel yad’, not his ‘yarmulke’ under his ‘shel rosh’! In cases of casts or bandages, a ‘shaila’ should be asked. (Note: Because there is a minority opinion – held by the Rashb”a - that is more lenient about ‘chatzitzos’ with regard to tefillin, particularly with the ‘shel rosh’!)