I am thinking of the cashflows structure of zero-coupon bonds. I am wondering whether they are usually issued below par or at par. It's more natural for me thinking that they are priced below par, but maybe I'm wrong. Thank you in advance.
There is only one cashflow for the zero-coupon bond. At maturity, it pays the par value.
They are always issued below par, as the buyer is paying the NPV for the bond that matures in the future.
Here is a brief reference at Investopedia.
A zero-coupon bond, also known as an "accrual bond," is a debt security that doesn't pay interest (a coupon) but is traded at a deep discount, rendering profit at maturity when the bond is redeemed for its full face value.
If you have a negative yield which cannot be ruled out nowadays the issueprice will be above par.