-tl- was like the first thing i learned about texture manipulation. But then you kinda had to really because HL1 had sooooooooooooooo many prefabs it was unreal. You seriously cannot live without it.
But then we are talking about when brush geometry made up 99% of detail work, and models were usually limited to animated objects such as creatures (xen lights, xen grass, xen jump pads). Even the Xen manta rays were brush work.
This is also when there was no model preview function and models lacked collision meshes so you had to produce an invisible brush the right shape, with the right texture material to produce the correct bullet ricochet sprites, using func_brush.
Used to have these little textures that said "metal" and "wood" on them for it.
Ahhh.... good times.