why the money supply does not change when one individual writes a check to another?

The money supply does not change when one individual writes a check to another because the act of writing a check does not create new money. Instead, it is simply a transfer of existing funds from one bank account to another.

When someone writes a check, it represents an instruction to their bank to transfer money from their account to another person's account. The check is then deposited into the recipient's bank account, and the funds are transferred electronically.

However, the total money supply in an economy is determined by the central bank and the banking system, not by individuals writing checks. The money supply is typically measured by the sum of physical currency (such as coins and banknotes) in circulation and the total deposits held by banks. These deposits are created through processes like bank lending.

So, while writing a check facilitates the transfer of funds, it does not increase or decrease the total money supply in the economy. It is important to note that the money supply can change through other factors such as the actions of the central bank, commercial bank lending, or other monetary policies.