Coupons are paid quarterly or semi-annually, but accrue continuously. If an investor sells a bond in the middle of a coupon period, they don't lose income for the days worked: the buyer compensates them for the accrued portion, and the new owner receives the full coupon on the payment date.
This explains the difference between clean and dirty prices. Quotes show the clean price—without accrued income. However, the full amount debited from the account includes accrued coupon income, so actual purchase costs exceed the quoted price.
This is neither a commission nor a loss: the accrued income paid is returned to the investor at the next coupon payment. The mechanism simply ensures fair income distribution among holders based on their actual ownership period.
Practical consequence: when comparing purchase costs for different bond issues, use the dirty price, not the quote. As the coupon payment date approaches, accrued income reaches its maximum, and the gap between clean and dirty prices approaches nearly one full coupon amount.