Assessment as a firm requires a certified partnership instrument with specified shares; noncompliance triggers assessment as association of persons. Assessment as a firm requires a written partnership instrument specifying individual partner shares and a certified copy of that instrument must accompany the first return seeking assessment as a firm; certification must be by all non minor partners or, after dissolution, by former partners and legal representatives. Continued assessment as a firm depends on no change in constitution or shares; changes require a certified revised instrument. Specified assessment failures or non compliance with the instrument requirements result in assessment as an association of persons.