# What does “st” mean in the context of OpenGL?

I've seen the term st come up a few times when looking at other OpenGL fragment shaders. However, I don't know what it stands for or what it's used for. Here's an example:

uniform vec2 u_resolution;

vec2 st = gl_FragCoord.xy/u_resolution;


I can understand that this is converting the pixel coordinates into normalized coordinates of 0.0 - 1.0 instead of 640 x 480 (for example).

I've also seen things like position.st.

What does it mean?

glTexParameter(..., GL_TEXTURE_WRAP_S, GL_REPEAT);

Where GL_TEXTURE_WRAP_S refers to wrapping in the S component of the texture coordinate.
In GLSL, you can swizzle with XYZW, STPQ, or RGBA. They all mean exactly the same thing. So position.st is exactly the same as position.xy. However, you're not allowed to combine swizzle masks from different sets. So position.xt is not allowed.