Postcards are single sheets of card stock that is stiffer than letter-writing paper. Postcards can have pictures, or not. Postcards usually do not have a return address but they do have a to address, postage and lack an outer envelope. In the United States we have strict sizes that postcards can be, if the card is larger then additional postage is necessary. If the postcard is smaller then it must be in an outer envelope that meets the minimum size requirements. Since I don’t carry a ruler with me I use a hard gauge to determine postcard postage requirements. Here is my hard gauge! Use it or modify it to suit the requirements of your country.
Enjoy!