Scribam, ergo salvum sum
A home-made Latin motto that speaks to the profound existential fear of all cautious solicitors. Something is only safe - only free from the black terror of the unknown that has a lair in the darkest reaches of every attorney’s heart - once it is written down.
The worst of it is that it isn’t even true: even if you write it down, the Leviathan still waits, and growls, and wants to eviscerate you, my young chicken.
The written articulation of a legal agreement — the paper it is written on, in other words — is no more than evidence of that agreement: it isn’t “the agreement” itself. A legal agreement is a metaphysical thing; a Platonic form: an intellectual construct, effable, but all the same unchained of any mortal, papery coil that might represent it in this world.
If, carelessly, the parties have failed to correctly document terms of the actual agreement between them, but they are, nonetheless, performing it, they can be held to it, as long as you can prove what the agreement actually was, whatever some flesh-weak piece of paper might say. A legally binding agreement might take the form of speech, non-verbal representations, winks, nods, grunts, insinuations, and outrageous untruths your salesperson has trotted out to the client while plying him with drink.
None of these things will be in your written contract, though be assured if your client is a little old lady the courts will expend considerable energy in finding out that they took place and formed a vital part of the consensus ad idem.