For deep gingival sheath preparation, shape of wedge should be:
Round
Triangular
Square
All of the above
Wedges are devices which are usually preferred for rapid tooth separation. These are used for tooth preparation and restoration.
Triangular wedge:

Most commonly used.
It has two positions — apex and the base.
Apex of the wedge usually lies in gingival portion of the contact area.
Base lies in contact with gingiva. This helps in stabilization and retraction of gingiva.
Used in tooth preparations with deep gingival margins.

Ref : Textbook of Operative Dentistry, Nisha and Amit Garg, 3rd edition, Pg no:209