We state the open and closed format. The open format marks its dimensions (width * height), when it is open/unfolded, while the closed format defines the format in folded form.
Also define the desired fold. We know a number of different folds and some classic and most common examples are presented on this page (above).
When stating the format, please specify whether it’s a portrait or landscape.
In the EU, we usually state formats in the following fashion: Width x Height. Example: 210 x 297 mm (portrait A4 format).
The English use their own method for naming the format: Height x Width (reverse), this is why it is important to also state if it’s a portrait or landscape in words.