Why does a triangle have 6 vertices?

Newbie & Debugging Questions
#1

I’m drawing learning a bit of LWJGL and I’ve made it up to the point where I draw a triangle with a FloatBuffer. The tutorial sets up the triangle’s vertices like this:


vertices.put(new float[] {
	+0.0f, +0.8f,    // Top Vertex
	+0.8f, -0.8f,    // Bottom-right vertex
	-0.8f, -0.8f     // Bottom-left vertex
});

If IRL math a triangle only has three vertices, why do they have 6 in LWJGL?