My students and I were discussing welding symbols. You have a single Vee-groove weld in a butt joint where one member is thinner than the other. Lets say one member is 3/8" thick and the other is 1/4" thick. How would you express the depth of preparation if you want there to be a 1/16" land on each piece? I know there is normally only one depth of preparation given, but in this case would or should there be two given?
Knowing you are an instructor and your class is probably not using a WPS, simply put a 45* angle dimension and a 1/16" root face detail on the board.
Depth of prep would result in 2 dimensions, which is why you would want to transition (2.5 to 1) the thick member to a thickness equal to the thinner piece. The detail only needs to be as strong as the weaker of the two elements. Anything else would result in overwelding the joint.
If a depth of bevel dimension is applied, it would be in relation to the thinner member joined @ 3/16".
I agree with DGXL concerning the detail of this joint. There are more guidelines shown in AWS D1.1 :2004 Section 2, paragraph(s) 2.7.1 & 2.16.1.1 -Structural Welding code to help with this.
Also see AWS D1.1:2004, Figure 2.2 (A) If welded from one side and has two different thicknesses and you transistion the weld surface at 2.5:1
or (B) you can use welding and chamfering at 2.5:1 of the thicker piece
or you can use (C) If you want to chamfer the thicker piece at 2.5:1 as DGXL explained.
I agree with DGXL concerning the detail of this joint. There are more guidelines shown in AWS D1.1 :2004 Section 2, paragraph(s) 2.7.1 & 2.16.1.1 -Structural Welding code to help with this. 
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