Origami often share common folding bases. Many diagrams start directly from a base. Therefore, it is important to know them.
As its name suggests, the preliminary base serves as a foundation for other origami bases. It combines two mountain folds along the diagonals of the square with two valley folds along the medians of the square.
This is the inverse of the preliminary base:

in 3D 

The frog base starts from the preliminary base:





