Seniority in the House, for Congressmen with unbroken service, depends on the date on which the members first term began. That date is either the start of the Congress (4 March in odd numbered years, for the era up to and including the 73rd Congress starting in 1933) or the date of a special election during the Congress. Since many members start serving on the same day as others, ranking between them is based on alphabetical order by the last name of the congressman.
