gml:Triangle 

gml:Triangle represents a triangle as a surface patch with an outer boundary consisting of a linear ring. Note that this is a polygon (subtype) with no inner boundaries. The number of points in the linear ring shall be four.
The ring (element exterior) shall be a gml:LinearRing and shall form a triangle, the first and the last position shall be coincident.
interpolation is fixed to "planar", i.e. an interpolation shall return points on a single plane. The boundary of the patch shall be contained within that plane.

Element Information

Model

gml:exterior
Children: gml:exterior

Attributes

QName Type Fixed Default Use Inheritable Annotation
interpolation gml:SurfaceInterpolationType planar optional

Source 

<element name="Triangle" type="gml:TriangleType" substitutionGroup="gml:AbstractSurfacePatch">
  <annotation>
    <documentation>gml:Triangle represents a triangle as a surface patch with an outer boundary consisting of a linear ring. Note that this is a polygon (subtype) with no inner boundaries. The number of points in the linear ring shall be four. The ring (element exterior) shall be a gml:LinearRing and shall form a triangle, the first and the last position shall be coincident. interpolation is fixed to "planar", i.e. an interpolation shall return points on a single plane. The boundary of the patch shall be contained within that plane.</documentation>
  </annotation>
</element>

Sample