In an orchestral score, each section of the orchestra gets a thick square margin bracket, e.g. the woodwind section.
Within these, thin square secondary brackets are used to group instruments in the same family, e.g. piccolo & flute.
When parts for the same type of instrument need to be written on separate staves, these are also grouped with a thin square bracket, but at a higher level (a tertiary bracket, if you will).
So, piccolo and divided flutes (in the flute family, in the woodwind section) will look like this:
Sometimes curly braces or thick square brackets are used for secondary/tertiary brackets instead.
In many of the orchestral scores that I have looked at, the string section groups the 1st & 2nd violins together, as well as the cello & contrabass (double bass) with secondary brackets.
So an undivided string section will have this form:
When any of these five of strings sections occasionally play divided (with divisi or with a solo / gli altri) tertiary brackets are used.
Grouping the 1st & 2nd violins makes sense because they are the same type of instrument. But, grouping the cello & contrabass together doesn't make much sense to me.
What is the function of secondary brackets for the cello & contrabass?
Contrary to this practice, Elaine Gould (in Behind Bars – The definitive guide to music notation, p. 518) says that grouping the 1st & 2nd violins with a secondary bracket (and by implication the cello & contrabass) is not good contemporary practice:
"1st and 2nd violins are regarded as separate sections and are not joined by a secondary bracket."
This means that Gould is suggesting that an undivided string section should take this form:
I don't find Gould's explanation at all satisfactory, especially since there are so many scores that do use secondary brackets in the string section.
Why is the practice of grouping the 1st & 2nd violins and the cello & contrabass, advocated against by Gould?