Deuteronomy 27:5

Ver. 5. An altar of stones— It does not seem probable, that the altar was built of the very same stones on which the law was engraven; for this evident reason, that the altar was to be built of stones, rough, unhewn, untouched by any tool; whereas some hard tool or instrument of metal was necessary to engrave the commandments: and as they could not have been engraved, so neither could they have been read easily, unless the surfaces of the stones were previously smoothed by art and labour. These stones are limited to two, according to Dr. Kennicott; because two large stones would be sufficient, and because it was most obvious for the Israelites to engrave the commandments upon two; in imitation of the two tables on which they had received them from God himself. It also appears clear, that only two were meant; the Hebrew word being frequently used in the plural, or, as some call it, the dual number, without the numeral for two being expressed at all. Thus it is used, Genesis 27:36 and thus abanim gedoloth, the very words used in the 1st verse of this chapter, are in the Latin versions of the Samaritan text, Exodus 20:18 translated TWO great stones. Kennicott. Diss. 2: See Houbigant on this verse.

See commentary on Deuteronomy 27:4