I think a number like 40 or 30 for these toothbrush heads is the number of "tufts" or tufts of bristles. Thus, the "40" head has about 40 tufts. As far as I know, the distance between the bars is not specified. Firmness is described as "soft", "medium" or "hard" (which is less popular these days). These 40 heads are fairly compact and slightly larger than 30 or 35 heads which are harder to come by in medium hardness. I like compact heads as I have a small mouth, they suit me well.
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