Quote:

No. Moreover, 'where' is not a question that enters the picture at all. The straps which bind the tefillin to the head or arm are different, so that each one in a set of two is worn only on the head or only on the arm [...] there isn't any leeway about where they are worn.



My question was about the tefillin/phylactery, not the straps, supposing, as you seem to, that these are to be regarded as separate objects.

Besides which, in stating that the tefillin may be worn "only" on the head or "only" on the arm you are not doing your point justice. What you mean to say is that the tefillin may be worn either on the head or arm, in which case "where" does potentially enter the picture especially when you consider that the differentiating attribute of the anklet and the bracelet -- otherwise being two identical ornamental hoops -- is where on the body they are worn.

Quote:

Phylactery is, I believe, of greek derivation, and tefillin of Hebrew.



What makes you think I couldn't find -- hadn't found -- these etymologies in whatever dictionary happened to be in reach? My question was simply : Is it a phylactery or a tefillin?

And consider :

tefillin

plural noun

collective term for Jewish phylacteries.

ORIGIN from Aramaic tepillin ‘prayers.’

and for phylactery :

phylactery

noun ( pl. -teries)

a small leather box containing Hebrew texts on vellum, worn by Jewish men at morning prayer as a reminder to keep the law.

phylactery

ORIGIN late Middle English : via late Latin from Greek phulakt?rion ‘amulet,’ from phulassein ‘to guard.’

Quote:

The Greek is the more accurate



My first thought was that they were synonyms, in which instance your instruction to choose a term according to a suppositious assessment of the tenability of their respective etymologies would hold; however, the definition for "tefillin" as "a collective term for Jewish phylacteries" suggests that there is more than one kind of phylactery; otherwise, why should there be a collective term for a single object?

Your tone of admonitory didacticism was promising, but ultimately leaves my question unanswered.