It is important to know the correct lingo when it comes to briefing a printer or designer with regard to folded print items. There are four basic folds that are the most likely to be used. These are Bi-fold (also known as single fold); Gate fold; Z-fold (also known as accordion or concertina fold); and Tri-fold (also known as letter fold).
Bi-fold is the fold that would be used to make a simple 4-page leaflet, usually with a front and back cover and opening to a spread of information. A typical set-up would be A3 folded to A4 for a small brochure or A4 folded to A5 for a leaflet. If the booklet has many pages and they are going to be bound (stapled) then the pages have to be in multiples of four, so 8, 12, 16, 20 etc.
Gate fold is less common but can be used to add impact to your printed items. As it is less frequently used it can have a much bigger impact than other folds as you can include a 'big reveal' piece of design behind the flaps. The example diagram above is a 6-page leaflet.
Z-fold (also known as accordion or concertina fold) is often sized as an A4 page folded into 3 equal pages so a 6-page leaflet in total. Each page in a Z-fold is the same size and this format can be used for the leaflet to free-stand on a display or in exhibition use.
Tri-fold (also known as letter fold) is the standard fold used for promotional leaflets. These are usually DL size, which is an A4 page folded into three. Unlike Z-fold the folds are slightly different measurements on each page so the internal pages of the leaflet fit behind the cover. These are 6-page leaflets and also the same format as a letter received in the post.