Sub-sub-brackets are a tertiary level of staff grouping with the same design as sub-brackets. They are positioned outside of both brackets and sub-brackets, allowing you to mark groups of staves within bracketed and sub-bracketed groups. Sub-sub-brackets can only appear as brackets in Dorico Pro.

Sub-sub-brackets cannot extend beyond their sub-bracket and cannot be shown on staves with a brace as either the primary or secondary group.

You can change how thick the lines of sub-sub-brackets are, their width, and their design in the Sub-sub-brackets subsection of the Design section of the Brackets and Braces page in Engrave > Engraving Options.