One thing I've noticed in a lot of beginners' drawings of the male body is that the abs often look the same in every single figure, portrait, or character that they draw. I've been guilty of this, myself, especially when drawing from my imagination rather than from a photo or a person.
But there are a lot of ways that abs can be different, apart from just the obvious "more defined" vs. "less defined" contrast. There are differences in the person's frame and overall proportions, there are differences in the connectivity and shapes of the muscles. These can be due to genetics, or due to differences in the way the muscles evolve because of how a person works out.
So as an exercise, I decided to take eight examples of extremely well-defined abs and sketch them, side-by-side, to see what I might learn.
I'll tell you what I learned in this exercise:
6-pack or 8-pack. All of the models have ridiculously low body-fat, but models for B, D, and G simply did not develop the "8 pack" look. This is partly physiology and partly the way they work out.
Ab symmetry. The abs on the left and right sides line up on models A, B, C and F, but they do not on models D, E, G or H. This also affects the look of the center vertical line, which sometimes looks like a "straight line" but in other cases looks very crooked.
Separation between chest and top abs. B and D have a very large separation between the chest and the top of the abs; A, C and E have a small but very well-defined gap; while in H there seems to be no real separation at all between the chest and top of the abs.
Width of the abs. B and E have very narrow abs relative to the width of their torso overall (you see a lot of thickness running down the SIDES of the abs), while C and G have very wide abs.
Flatness of the abs. D and F have very flat abs, which I tried to get across with the shading (notice that the shaded areas are very narrow), while A, C, E and G all have very rounded and protruding abs (notice that there are thicker amounts of shading outlining the muscles).
Torso shape. C and G have very V-shaped torsos, which also affects the way that the abs look. E, D and A are at the opposite extreme.
One last interesting thing: what can you tell from a person's abs? Can you tell just by looking at these sketches that model G is 5'9" and models D and E are both 6'3"? Can you tell that models F and H have a more square jaw and models A and D are more "boyish" looking?
It's interesting to try to find relationships. And to think about how knowing this stuff can help you to draw more realistic "characters" out of your imagination.
Just a quick little thing about contrast c: Very vague and small, but maybe it will be useful xD I should mention that this really isn't the best example to use, because the whole concept of values really mostly applies to realistic works. But eh, it was late and I was lazy sooo- take what you can get from this.