My sources say they can also be written: 'biavtal till exemplvis ett hyresavtal' - Juridikens termer, Almqvist & Wiksell.

If verbal, then - unless biblical - oral covenant is unusual, as a covenant normally has to be contained in a written deed. Oral undertaking is also specific to some professions i.e. Solicitors/Attorneys in the UK and Lloyds insurance underwriters , both of whose oral promises are as good and enforceable as a written contract.

Quote from a Canadian case:

Subsequent to *oral assurances* that buckwheat would smother weeds, farmers entered into a contract which contained the words: "no warranty ... pertaining to the seed sold ... and will not in any way be responsible for the crop." The buckwheat planted did not act as a weed control and the crops were smothered and destroyed. The British Columbia court decided that the evidence showed an *oral warranty* that defeated the "the strong presumption" in favour of the written contract. It then issued a series of "comments" about the parol evidence rule:
The rule is a rule of evidence. One can introduce evidence to prove a *collateral agreement* provided it does not contradict the written agreement because it cannot be that the parties would agree to two contracts which disagree with each other. Since the written contract was demonstrably made, "reasons requires one to conclude that the oral one, contradicting it, was never made."
The principle is not absolute; it is "not a tool for the unscrupulous to dupe the unwary." The court can look for evidence of an intention to create a binding oral agreement such as it did, albeit unsuccessfully, in the Hawrish and Bauer cases summarized above.
"If the contract is induced by an oral misrepresentation that is inconsistent with the written contract, the written contract cannot stand."
"The principle does not apply with equal force where the *oral representation* adds to, subtracts from or varies the agreement recorded in (as opposed to contradicting) the document."
The parol evidence rule is a "strong" presumption that a written contract represents the whole agreement between the parties; "strongest when the oral representation is alleged to be contrary to the document and somewhat less strong when the oral representation only adds to the document."
The presumption is not as strong for a printed form document "though it would be a strong presumption in both cases."
The presumption would also be "less strong where the contradiction was between a specific oral representation and an .. exclusion clause that excludes liability for any oral representation whatsoever."