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Tefillin Gassos are shaped out of one thick piece of cowhide. The Battim are very strong and long lasting. They will retain their shape and squared corners throughout many years of constant use. Even when they become old and worn they can be restored to look as good as new.
If you are right-handed you wear tefillin on your left hand (your weaker...
Peshutim Mehudarim are made of many pieces of parchment that are fastened together in a special manner to ensure their Kashrut. Peshutim are the least expensive Tefillin. If you're right-handed you wear tefillin on your left hand (your weaker hand) and vice versa.