Vertex shaders allow input layout qualifiers on input variable declarations.  The layout qualifier identifier for vertex shader inputs is:

layout (location = integer-constant)

For example, to use the generic vertex attribute 0, representing vertex position, in the vertex shader:

layout (location = 0) in vec3 vPos;

This will establish that the vertex shader input vPos is copied in from location number 0.  Recall the generic vertex attribute is configured when setting up the VBO, through the call glVertexAttribPointer.

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