$100 appears in your account. That’s one part. The other part depends on why.
* you moved money from another account, the double is -100 in that account.
* you sold stuff, +100 in income.
* you borrowed some money, +100 in ‘debt’.
In a physical book each of these categories would have a left and right column, and each transaction has numbers in one left and one right column. Or in many columns but the sums of left vs right columns must be the same.
$100 appears in your account. That’s one part. The other part depends on why.
* you moved money from another account, the double is -100 in that account.
* you sold stuff, +100 in income.
* you borrowed some money, +100 in ‘debt’.
In a physical book each of these categories would have a left and right column, and each transaction has numbers in one left and one right column. Or in many columns but the sums of left vs right columns must be the same.