Passphrases have many uses, such as serving as seeds for passwords. User-created passphrases are easier to remember, but tend to be less secure than ones created from words randomly chosen in a dictionary. This paper develops a way of making more memorable, more secure passphrases. It investigates the security and usability of creating a passphrase by choosing from a randomly generated set of words presented as a two-dimensional array. A usability experiment shows that participants using this method achieved 97% to 99% of the maximal theoretical entropy and commited fewer than half as many memory mistakes as a control group with assigned passphrases. It also shows that their choices are affected by word familiarity and weakly by the word’s position in the array. Prompting a person with random words from a large dictionary is an effective way of helping them make a more memorable high-entropy passphrase.