This is a main category requiring frequent diffusion and maybe maintenance. As many pictures and media files as possible should be moved into appropriate subcategories.
The following special combining grapheme joiner (CGJ) diacritic is invisible, and typically used between two other diacritics having different combining classes when they may interact graphically in a distinctive order, it allows overriding their default rendering order implied by their Unicode normalisation (which reorders diacritic with the lowest combining class before all others having a higher or equal combining classes). It is useless and should not be encoded between diacritics that are encoded in ascending order of combining classes, or that don't interact graphically because they are at unrelated positions. But note that some graphical diacritics may have their default position altered implicitly on some base letters (including between letters within the same casing pair), and insertion of CGJ may be needed when changing the letter case.
The following combining Greek perispomeni diacritic (combining above) is a Greek-specific form of circumflex for the rising-falling accent, similar to a combining tilde (U+0303), but with an alternative glyph similar to an inverted breve (U+0311).
The following combining Greek koronis diacritic (combining above) is canonically equivalent to the combining comma above (U+0313) with adjustment of its position relative to a base Greek letter.
Use of the following combining Greek dialitika tonos diacritic (combining above) is discouraged in favor of the combining diaeresis (U+0308) followed by the combining acute accent (U+0301).
The following combining Greek ypogegrammeni (or iota subscript) diacritic has special casing issues. See also U+037A for the lowercase spacing modifier letter, and U+0399 for the capital letter.
The following double-width diacritics are intended to be used between two base characters or combining sequences, so that such diacritic extends to cover both bases.
The following diacritics (combining above) are letters written directly above other letters. They appear primarily in medieval Germanic manuscripts, but saw some usage as late as the 19th century in some languages.