Custom used to be one of the most important sources of law but has now, to a large extent, been superseded by statute and precedent. The acceptance of custom as law is far more prevalent in emergent legal systems than it is in mature legal systems.

There are two main grounds on which custom is accepted as law:
1. A state, when it begins to administer justice, respects what society acknowledges and approves of.
2. If there is a widely accepted custom, it is rational and in the interest justice that the custom be continued to be recognised and accepted, in this case, by its incorporation and/or recognition in the law.

Legal Customs

Legal customs are those customs whose authority is absolute under the law. They operate without the medium of a contract and can therefore be directly enforced.

Requirements for Custom to be accepted as Law:
1. Duration (Legal customs should have been existence since time immemorial but there is no such requirement as to duration with reference to conventional customs.)
2. Uniformity
3. Consistency
4. Generality
5. Universality

Conventional Customs

The authority of conventional customs depends on their acceptance and incorporation into agreements by parties which agree to be bound by them. In contractibus tacite veniunte ea quae sunt moris et consuetudinis.

Law originating from the acceptance of certain customs by merchants and the incorporation of those customs into law has resulted in the creation of what is known as ‘law merchant’ (and is a type of conventional law).

Conventional customs pass through three stages historical before they are accepted as law:
1. The custom in question is proved by witnesses.
2. The courts take judicial notice of the custom, and it can therefore be proved using precedent: its immediate source.
3. The custom is incorporated into statutory law.

If conventional customs conflict with Statute, they are admitted only to the extent that statutory law allows their admission. The conventional custom cannot derogate from law any further than the law itself allows. If the law does not allow parties to do something by contract, parties cannot do that thing by establishing a custom which allows it.

If a later conventional custom conflicts with an earlier custom which has been judicially noticed, the earlier custom will prevail.

General Customs

In England, general custom throughout the realm is Common Law. In Blundell v. Caterall, 1821 Best, J. said, “the practice of a particular place is a custom. A general immemorial practice through the realm is the common law.” While this may have been true of English Law in its infancy, the fact of the matter is that, as Salmond pointed out, ‘the Common Law of England has long since ceased to be customary law and has become a body of case law instead’.

Local Customs

Local custom can, in no case, be become part of the law of the whole State. Its operation is confined to a particular local area. For a local custom to be valid, it must be:
1. Reasonable
2. Consistent with statute
3. Observed as of right
4. In existence since time immemorial.

1. Reasonableness

Malus usus abolendus est.
To be valid, custom must be reasonable. This is not to say that it may be disregarded by Courts whenever they think that it is not very wise, it merely means that courts may choose not to accept a custom where it is obviously against public policy and justice.

2. Consistency with Statute

A custom cannot conflict with a statute in English Law. However, Roman Law and legal systems on the Continent follow the maxim ‘Lex posterior derogat priori’.

3. Observance as of Right

Nec vi nec clam nec precario.
‘Right’ over here has got nothing to do with moral right. It means that the custom should have been observed:
1. Openly
2. Without recourse to force
3. Irrespective of the permission of those whom it affected adversely.
e.g. Merchet was always considered undesirable by both tenants and the Church but was a well established custom in many manors nonetheless.

4. Duration

For a custom to be valid, it must have been in existence since time immemorial. As Littleton put it, “No custom is to be allowed but such custom as hath been used by title of prescription, that is to say from time out of mind. ‘Time out of mind’ in Roman and canon law means a time so remote that no man can remember or give evidence concerning it.

In England, however, human memory has become distinguished from legal memory. The Statute of Westminster passed in 1275 that ‘time out of mind’ extended only as far back of the accession on Richard I in 1189 – this law has not changed and the law now imputes that human memory extends to over eight hundred years!

Reference: Salmond on Jurisprudence, Bodenheimer