The post office sells postcard stamps that are cheaper than regular first-class stamps. But there are rules that limit the size and shape of postcards that can be used with these stamps. If you don't follow these rules, your postcard will have to be mailed with a regular first-class stamp. There are other requirements about placement of postage, address and other features on the mailing side of the card that you need to know about as well.
Postcards must be rectangular in order to be mailed with a postcard stamp. The longest side of the rectangle has to be horizontal when you are reading the address, even if the text on the other side of the card goes the other way.
There is both a minimum and maximum size. If your card is too small or too big, you will have to use a first-class stamp. The minimum size is 5 by 3-1/2 inches, and the maximum size is 6 by 4-1/4 inches. There is also a minimum and maximum thickness, which means you can't make a postcard by cutting paper to postcard size, and you can't send a postcard using corrugated cardboard. The minimum thickness is 0.007 inch and the maximum is 0.016 inch thick.
The postage goes in the upper right hand corner of the side with the address on it. Nothing else but postage should be in this corner. The space left clear for postage is 1-1/4 inches from the top and from the right-hand edge. Don't write anything in this space, and if you are having postcards printed, don't put anything in that space.
The mailing address has a specific location also. There is a little more leeway with this, but there are still some rules. Measure 2-3/4 inches from the right-hand edge on the mailing address side of the card. The area to the right of that line must contain the state and Postcode part of the address. Also, the mailing address should not be above 2-3/4 inches from the bottom of the card. This area is like the postage area: nothing but the mailing address should be printed on this part of the postcard. Any slogan or other marketing information should be somewhere else on the card.
There is one more area and it's easy to forget. Measure 5/8 inch from the bottom of the card. This part of the card, on the mailing address side, must be left open for the bar code. This area goes all the way across the bottom of the card. "But I'm not using a bar code," you say. It doesn't matter. The post office may affix a sticker with a bar code, or print one right on your card, during the mailing process. If you have printing in this area, it will be obscured or rendered illegible, which can increase delivery times.