By tracing shares to the individuals who hold them and adding up the percentage held by people who carry the treaty nationality. At least fifty per cent must be in treaty-national hands, and a co-owner who has become a U.S. lawful permanent resident no longer counts toward that half.
The half that counts is counted person by person
The arithmetic sounds simple and produces surprises. Ownership is traced through to natural persons, so a corporate shareholder is opened up rather than treated as a unit, and every layer is followed to whoever ultimately holds the shares. Nationality is then applied to those people.
Hypothetical example: two partners buy a lesson and boarding barn, one holding sixty per cent and the other forty. Both were born in Québec and both hold Canadian passports, but the second partner became a U.S. lawful permanent resident four years ago.
Her forty per cent does not count toward treaty ownership, because a treaty national who has taken permanent residence in the United States is treated as domiciled there for this purpose. The first partner's sixty per cent still carries the enterprise past the threshold, so the case survives. Move the figures to forty-five and fifty-five and it does not.
The lesson is to run the calculation before the shares are issued rather than after, since restructuring an ownership split once money has moved is expensive and can itself raise questions about who really controls the business.