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It is not a matter of aligning the decimal points, but aligning the place value columns so that the ones are under each other, the tens are under each other, the tenths are under each other, etc which is the proper way to subtract. As the decimal points are between the ones and tenths columns, with those place value columns aligned, the decimal points are aligned.Aligning the decimal points is an easy way to remember to align the place value columns, as with the decimal points aligned all the place value columns are automatically aligned.
they are expressions
Each column has a place value that is ten times the place value of the column to its right.
In the decimal system the place value of any digit is ten times that of the digit to its right. If using other bases a similar relationship applies. In the octal (base 8) system, for example, place value of each digit is eight times that of the digit to its right.
The commas between numbers in place value are commas.