In my opinion, Artscoll’s most important contribution is the Artscroll Shas which has enabled and encouraged thousands to learn gemorah. it’s maintainance of the tzuras hadaf and it’s own commentary make it a superior learning vehicle for all.
With it’s siddurim, Artscroll greatest contribution is the elimination of errors and poor typesetting that had crept into most of the earlier mass printed siddurim and the re-inclusion of tefilos and parts of tefilos that had been eliminated for political reasons.
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