>>    (set-fontset-font "-*-Terminus-*-fontset-startup"
>>                      '(0 . #x3FFFFF)
>>                      "Terminus")
>> 
>> Looks very much like (setq a a), but works like a charm!
>
> Are you sure this doesn't get in the way when you want to display a
> character not supported by that font?

No, I am not.  But how it could if I did mess with ‘fontset-default’?  Anyway, let us consider that example:

| Unicode charset: “Eĥoŝanĝo ĉiuĵaŭde”, “Γειά σας”, “שלום”, “Здравствуйте!”

Before (note that the fallback font is inconsistent even linewise — the first opening quote is different from others):