A contract does not need to be written down to bind the people who made it. What the law asks is whether there was an offer, an acceptance, something given in return, and an intention to be bound, not whether any of that was typed and signed. A great many everyday arrangements meet that test without a page of paper between them: a builder’s undertaking to finish work by a date, a loan between two families, a standing understanding between a supplier and a shop.
What changes without a document is the question a court has to answer. Where the agreement is written, the dispute is usually one of construction: what a clause says, what it means read with the rest of the document, and what the parties did afterwards. Where nothing was written, the same terms have to be pieced together from everything else made at the time: correspondence, invoices, ledgers, part-payments, delivery records, and the conduct of both sides. None of this is a weaker kind of proof in principle; it is proof that has to be gathered rather than read off a signature page, and it is strongest when it was made in the ordinary course of the dealing rather than assembled after a dispute has already begun.
A smaller set of agreements is different in kind, not just in proof. A sale of immovable property has no effect at all unless it is by a registered instrument, whatever the parties orally agreed; certain guarantees, and a lease beyond a year, need writing and registration to bind anyone. For these, an oral understanding is not merely hard to prove: it is not a contract the law will act on, however clearly it is remembered by everyone concerned.
For most other agreements, the practical question is what to keep. Texts, emails and dated messages; receipts and bank transfer references for a loan; a delivery note signed on receipt: these are what an advocate asks for first when an oral agreement is disputed, and they are worth keeping from the day the arrangement is made, not from the day it turns into a dispute. See Contract & civil disputes.
This note describes the general position only. Whether a particular understanding is enforceable, and what evidence it needs, depends on the kind of transaction and the facts of the case, and nothing here is legal advice.