I would like to look into the details of the usefulness of double entry, which enables us to recognize the duality of transactions.

Money consumes, Expense occurs.

The key is how we can express this increasing and decreasing at the same time.

The dual entry expresses this with using two columns, right hand side and left side.

At first we put 100 JPY on left side.

When you buy a bottle of water with 100 JPY, you need to put 100 JPY on right side and 100 JPY water expense on left side.

Now we describe this. In other words, money on right side move to right side, so we can say it has gone. At the same time we recognize 100 JPY expense.

The way of this is called as journal entry and it is as follow.

Water expense 100 JPY / Money 100 JPY

This duality is the usefulness of double entry.