When designing a Tri-fold letter size brochure it's important to take into account that the panel sizes are not all equal. In the case of letter sized brochures the front and back covers should be sized to 3.6875", and the remaining panel should be 3.625". This takes into account that one panel folds inside the other, thus it needs to be shorter so it won't bind up when folding.
The same concept applies to any folding brochure or kit cover where panels fold inside other panels. Brochures are usually printed on text weight papers, so the amount needed to compensate is small (.0625"). When using heavier stocks the shorter panels may need to be shorter by more than 1/16".
As always, if you have any questions about your folding project and how to size the panels correctly, please give us a call!