Most letters look better when they are written with some changes in pen angle. This style depends on them.
B old letters pose challenges all their own. Quality of line is not as important as it is for lighter weights. But bold letters need a striking silhouette. There are restrictions as well. When the characters reach certain boundaries, they can't get any fatter.
The numeral 8 can look all right without a bold middle. The three black horizontals of the letter E are a serious limit. A percentage symbol is worse still. How bold can you make it before the white countershapes become too small? This in turn restricts the maximum weight of all the other characters.
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