It separates the place values with positive power of ten from those with negative powers of ten. In other words, it separates the whole part of numbers from their fractional parts.
Not in the way we that would use a decimal point today. But they did use points to signify fractions of 12.
You can never use more than one decimal point.
There should be no points after the decimal point.
87.656 87.66
Decimal points aren't positive or negative. Decimals can be either.
Where a number is showing some decimal points and you want to decrease the amount of decimal points it shows, you can use the decrease decimal button to do it. Each time you click it, it will remove one decimal place until you have the amount you want or all are gone. There is also a corresponding increase decimal button, which does the opposite.
Put the numbers in a column with the decimal points aligned. Add the numbers ignoring the decimal points. Insert the decimal point in the answer exactly below the column of decimal points in the numbers being added (summands).
A number with two decimal points cannot be a decimal number.
Write the numbers in a column so that all the decimal points are one below the other. Put a decimal point in the answers underneath the coumn of decimal points. Then add the numbers ignoring the decimal points.
Unlike addition, there is no need to line up the decimal points. Just multiply ignoring the decimal points. When you are done, move the decimal point of the result to the left equal to the number of digits after the decimal point for both multiplicands.
A decimal number can't have two decimal points in it
When adding decimal numbers, the decimal points should be lined-up.