A customer opens a margin account by purchasing 100 shares of ABC at $60 per share, depositing the 50% Regulation T requirement. The stock rises to $80 per share on the next day and then falls to $60 per share on the day after. The account will now show:

Account Balance in margin account:

Investment = $6,000 (100 x $60)

The customer's account will first increase with an unrealized gain of $2,000 ($80 - 60 x 100) on the next day.  It will then decrease with an unrealized loss of $2,000 ($80 - 60 x 100) on the day after.  This cancels the earlier unrealized gain.

Explanation:

The customer's investment will now show a balance of $6,000 with a contra account showing a debt of $3,000 for the balance of the Regulation T margin account.  According to investopedia, "A margin account is a brokerage account in which the broker lends the customer cash to purchase stocks or other financial products.  The loan in the account is collateralized by the securities purchased and cash, and comes with a periodic interest rate."