The table below presents an aligned extract of verselets in Gn42-27c Both verselets discuss the money found in the knapsacks of the brothers of Joseph. The alignment justifies the Rashi assertion that The words for bag, knapsack,sack mean the same and can be used interchangably.

Verse	Text of Verse	Rashi comment
Gn42-27	And as one of them opened his bag to give his ass provender in the lodging-place,	The underlined words bag and knapsack are used interchanably.
Gn42-27	he saw his money; and, behold, it was in the mouth of his knapsack.	The underlined words bag and knapsack are used interchanably.

Advanced Rashi: But perhaps there is a difference in the nuances of the words. The following further almost parallel verses show this that the words - bag, knapsack- are basically interchangeable.
• Gn47-25, Then Joseph commanded to fill their bags with grain, and to restore every man’s money into his bags, and to give them provision for the way; and thus did he to them.
• Gn47-27, And as one of them opened his bag to give his ass provender in the inn, he saw his money; for, behold, it was in his knapsack's mouth.
• Gn47-28, And he said to his brothers, My money is restored; and, lo, it is in my knapsack; and their heart failed them, and they were afraid, saying one to another, What is this that God has done to us?
• Gn47-35, And it came to pass as they emptied their bags, that, behold, every man’s bundle of money was in his bag; and when both they and their father saw the bundles of money, they were afraid.

Perhaps the words differ in their etymology. Aleph-Mem-Tauv-Cheth emphasizes the tightening of supporting strings (from Mem-Tauv-Cheth the securing of a place, such as a tent, with tightening strings) while Sin-Kaph emphasizes the fact that the bag is used for intense carrying (From Shin-Kaph-Kaph the market place.). However since all intense carrying bags involve tightening of strings to secure contents the words would then be interchangeable.