Русско-англо-русский юридический словарь

unwritten constitution: a constitution not embodied in a single document but based chiefly on custom and precedent as expressed in statutes and judicial decisions (merriam-webster.com)

The UK constitution is often described as an 'unwritten constitution', but it is best described as 'partly written and wholly uncodified' (Budge et al, 1998). (bbc.co.uk)

A constitutional monarchy is a form of monarchy in which the sovereign exercises authority in accordance with a written or unwritten constitution. (Wikipedia)

The unwritten law here is that most customers will forgive you the first time you screw-up. (Forbes)