Under a family-first order, should a lowercase von/van den opening the name start the family? van den Berg Cornelia gives family van #670

rules.md#P1 folds a particle OPENING the name into the family (under a family-first order: the particle run plus one name unit), but only a never-given particle. An ambiguous one (van, von, bin, da, den, …) "keeps whatever reading its position gives it" — and under FAMILY_FIRST that position is the family slot, so the particle becomes the whole family and the surname it belongs to becomes the given name:

parse under FAMILY_FIRST           family        given
van den Berg Cornelia          →   'van'         'den Berg Cornelia'
von Wolf Ada Cornelia          →   'von'         'Wolf'   (middle 'Ada Cornelia')
bin Stromae Elizabeth Windsor  →   'bin'         'Stromae'
de la Vega Juan                →   'de la Vega'  'Juan'   (never-given: folds today)

Under a family-first order the ambiguity P1 protects (particle or given name?) is already settled by the declaration — the opening slot is the family. What is left is whether the particle is a one-word surname by itself or the start of one, and the first reading is the bad case in nearly every name.

The canary: some ambiguous particles are also family-first surnames — do, le, la are Vietnamese Đỗ, Lê, Lã — and Le Van Thanh (FF) must stay family Le, given Van Thanh. So the fold can't simply widen to every ambiguous particle.

Candidate statement: under a family-first order, an ambiguous particle opening a mixed-case name and written lowercase folds as a never-given one does; a capitalized one keeps its positional reading. Case-dependent readings have precedent in CONJUNCTIONS_AMBIGUOUS (#383/#479), also gated on a mixed-case name.

Measured (simulated by removing the particle from particles_ambiguous for qualifying names, master 1f0b374, 2026-10-13):

• CLDR corpus: 39 of the 1,294 FAMILY_FIRST renderings move, 16 become fully agreeing, none that agree today stop agreeing. Most of the other 23 now have the right family but still disagree for another reason (a title written mid-name, von Brühl Prof. Dr. Ada …; a second surname; a comma).
• Boundaries:

van Gogh Vincent    FF  →  family 'van Gogh', given 'Vincent'   (today 'van')
van Gogh            FF  →  family 'van Gogh', given ''          (as `de Mesnil` reads under FF)
Le Van Thanh        FF  →  family 'Le', given 'Van Thanh'       (unchanged: capitalized)
Do Van Thanh        FF  →  family 'Do'                          (unchanged)
le Van Thanh        FF  →  family 'le Van Thanh'                (MOVES — accepted cost)
van                 FF  →  family 'van'                         (nothing to fold into)

Accepted consequence: a lowercase-written family-first surname that is also an ambiguous particle (le Van Thanh) folds into the family. Writing a Vietnamese surname lowercase in an otherwise capitalized name is not the convention; the capitalized form is unaffected.

Open questions:

• Same for FAMILY_FIRST_GIVEN_LAST? (P1 treats the two family-first orders alike, so presumably yes.)
• The default order stays out of scope — van Gogh keeps given van there.
• A capitalized Van der Merwe Jan Koos (CLDR af) keeps family Van under this candidate. Is a capitalized run of two or more particles evidence enough to fold too?

Excluded: about 30 more CLDR renderings show the same symptom because the particle is not vocabulary at all — transliterations of "von" in other scripts (фон, вон, φον, פון, فون, ფონ, フォン, 폰, ቮን, …). These are a rare edge case for the cost they would carry (the Chinese one, 冯/馮, is a top-30 surname), so they stay out. A few more are CLDR test data placing a surname in surname-prefix (রহমান, Syed A).

Found through the CLDR corpus (cldr_agreement.json, foreignFull/nativeFull in surnameFirst order).