Take this book of the law,.... Not Deuteronomy only, but the whole Pentateuch:and put it in the side of the ark of the covenant of the Lord your God; not in the ark, for there were nothing there but the two tables of stone with the ten commands on them, 1Ki 8:9; but on one side of it; the Targum of Jonathan says, it was"put in a chest (or box) on the right side of the ark of the covenant;''which is very probable.
Jarchi observes, that the wise men of Israel are divided about it in the Talmud ; some of them say there was a table (or ledge) that stood out from the ark without, and there it was put; others say it was put on the side of the tables of the law within the ark; the former are in the right:that it may be therefore a witness against thee; when they fall into idolatry or any other sin, a transgression of any of the laws therein contained.
T. B. Bava Bathra, fol. 14. 1.