Assessment as a firm: instrument-based partnership and specified partner shares determine firm assessment; non-compliance disallows partner payments. A firm is assessed as a firm if the partnership is evidenced by an instrument and partner shares are specified; a certified copy of that instrument must accompany the firm's return when assessment as a firm is first sought, certified by all partners (not minors) or by former partners/legal representatives after dissolution. Continuity of assessment follows where there is no change in constitution or shares; changes require furnishing a certified revised instrument. Specific non-compliance by a firm triggers assessment rules that disallow deductions for payments to partners and alters their tax treatment under the Act.