These are agreements made by a couple who are about to get married. This may not seem very romantic but it may be sensible, for example, if either wants to protect assets for their children from a previous relationship or protect assets that they have inherited. Pre-nups are not strictly binding on the court if there is a later divorce but will be followed unless unfair. To make the agreements as strong as possible there should be full financial disclosure by both and the agreement should be prepared well in advance of the wedding to avoid the suggestion of undue pressure. The agreement needs to consider what you both brought into the marriage: family home; property given or inherited; money in joint accounts; debts; pensions; money saved during the marriage; points when the agreement should be reviewed; if either should receive maintenance and if there are any children during the marriage.
If you are already married, you can enter into an agreement about what you intend to do with money and property if the marriage were to end. This would often be if there was not enough time before the marriage to prepare a pre-nuptial or there had been a separation and reconciliation. Fairness is again the main consideration. The points that apply to pre-nuptials also apply. There needs to be full financial disclosure by both. The post-nuptial will deal with exactly the same things as listed in the information above about pre-nuptial agreements.