According to Cicero’s rhetorical treatise the Ad Herennium, the classical art of artificial – as opposed to normal, or natural – memory comes from a systematic visualization of places and images [ex loci et imaginibus]:
the places are very much like wax tablets or papyrus, the images like the letters, the arrangement and disposition of the images like the script, and the delivery is like the reading.
You should first memorise a great many places, arranged in a very particular order. The easiest way of doing this is to visit a large public building, and commit every corner and room of it to memory. To keep the order of these sites clear in the mind, Cicero advises giving every fifth location “some distinguishing mark:”
We may for example mark the fifth locus with a golden hand, and place in the tenth the image of some acquaintance whose name is Decimus [= tenth].
The next step is to place images in each of the places, images which should be as striking and memorable as possible:
things immediate to our eye or ear we commonly forget; incidents of our childhood we often remember best. A sunrise, the sun’s course, a sunset are marvellous to no-one because they occur daily. Solar eclipses are a source of wonder because they occur seldom.[*]
We ought, then, to set up images of a kind that can adhere longest in memory. if we set up images that are not many and vague but active; if we assign to them exceptional beauty or singular ugliness; if we ornament some of them, as with crowns or purple cloaks, or if we somehow disfigure them, as by introducing one stained with blood or soiled with mud or smeared with red paint, or by assigning certain comic effects to our images, that will ensure our remembering them more readily.