These letter folds are almost identical. They have different additional crease patterns with the easier version having a lot more of them.
Here is the easier version (Type One).
The angled flaps are derived by folding a diagonal from the edges of the outside columns and then reflecting a marker crease from the orthogonal direction.
And here is the cleaner but slightly more difficult version (Type Two).
This time the angled flaps are derived from diagonals that are creased between sections that are an eighth of the overall height or width. To that end the pre-creasing divides the sheet into eight in both directions.
Since publishing this blog I discovered that this design was first invented by Romie Halabaso from the Philippines. The diagrams and crease pattern do not appear to be in the public domain so I cannot tell whether our patterns are identical or not.
You will find Romie's letter fold here:
http://www.origami-resource-center.com/letter-envelope.html
