Must tefillin be written with the right hand, and are tefillin written with the left hand invalid?

Synopsis

The Mechaber rules tefillin must be written with the right hand even for an ambidextrous person. Tefillin written with the left hand are invalid if valid right-hand-written tefillin can be found. A left-handed person's left hand counts as their right.

Early Acharonim
Late Acharonim
Lenient
Strict
Lesser cited
Major authority

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