No one knows who made the drawing. Likely the work of an eighth-century monk, perhaps a member of an esoteric Buddhist cult traveling the Silk Road, it long sat forgotten in a walled-off library in China’s Mogao Caves. When the library was uncovered in 1900, the drawing — lifted from a trove of religious manuscripts — had aged well. Its subject is timeless: a pair of human hands.¹

The hands are disembodied, perched on lotus petals, palms facing the viewer. Their fingers — vigorous and elegant — are annotated with Chinese characters: the lowest tier of characters, on the tips, names each digit; above that, a second row gives the five Buddhist elements: space, wind, fire, water, and earth; and a final tier, floating upwards as if on kite strings, lists the ten virtues — among them meditation, effort, charity, wisdom, and patience. The drawing illustrates a mnemonic system, a way of projecting knowledge onto the hands so it can be studied, memorized, and stashed in a pocket.

Around the same time this mnemonic was made, another monk — in a Northumbrian monastery, halfway around the world — was developing a different system of manual knowledge. His name we do know: Bede. In 725, he published a treatise, The Reckoning of Time, in which — alongside discussions of shadows, moonlight, and the solstices — he laid out a method for determining when Easter would fall on any given year.² This may sound like a trivial exercise, but, for Christians at the time, it could hardly have presented a more important or vexing problem. ※

To find the date of Easter — which falls after the Northern Hemisphere’s spring equinox, on the Sunday immediately following the first full moon — one needs to reckon with planetary rhythms, which Bede mapped across his hands. The five fingers, he observed, contain fourteen joints, plus five nails — nineteen landmarks in all. This number tracks the metonic cycle: how many years it takes for the moon to return to the same phase on the same calendrical day. The joints of both hands taken together, minus the nails, gives you twenty-eight landmarks: the approximate length in years of a full solar cycle. In this way, Bede noted, the hands can “readily hold the cycles of both planets”.³ Beyond this basic set-up, he left the details obscure and didn’t bother to include an illustration. (The technique, Bede wrote, is “better conveyed by the utterance of a living voice than by the labor of an inscribing pen”.⁴) But his system — known as a computus digitorum, or simply computus — found an appreciative audience. It was widely circulated and adapted, and would remain a cornerstone of Christian learning for centuries.

These two systems — perhaps the earliest examples of manual mnemonics — come down to us only in outline. And yet we have little trouble recognizing their appeal. They seem to spring from an impulse that transcends time and place, a deeply human drive to reach for props to help us reason and remember. “When thought overwhelms the mind”, the psychologist Barbara Tversky has written, “the mind puts it into the world”.⁵ In the case of hand mnemonics, we put those thoughts out into the world, in a sense, but also keep them within easy reach.

In the beginning, the hand was just a hand — or so we can imagine. It was a workaday organ, albeit a versatile one: a tool for grasping, holding, throwing, and hefting. Then, at some point, after millions of years, it took on other duties. It became an instrument of mental, not just menial, labor. As a species, our systems of understanding, belief, and myth had grown more elaborate, more cognitively overwhelming. And so we started to put those systems out into the world: to tally, track, and record by carving notches into bone, tying knots in string, spreading pigment on cave walls, and aligning rocks with celestial bodies. Hands abetted these early mental labors, of course, but they would later become more than mere accessories. Beginning roughly twelve hundred years ago, we started using the hand itself as a portable repository of knowledge, a place to store whatever tended to slip our mental grasp. The topography of the palm and fingers became invisibly inscribed with information of all kinds — tenets and dates, names and sounds. The hand proved versatile in a new way, as an all-purpose memory machine.

The arts of memory are well known, but the role of the hand in these arts is often overlooked. In the twentieth century, beginning with the pioneering work of Frances Yates, Western scholars started to piece together a rich tradition of mnemonic practices that originated in antiquity and later took hold in Europe.⁶ The most celebrated of these is the “memory palace”. Using this technique, skilled practitioners can memorize vast collections of facts by nesting them in familiar places (or “loci”): the chambers of a building or along a well-known route. (To make these places more memorable, a bizarre image is often added to each one, the more jarring the better.) It is an odd omission that hand mnemonics are rarely mentioned alongside memory palaces. Both techniques are powerful and broadly attested. Both are adaptable, able to accommodate whatever type of information one wants to remember. And both work by similar principles, pinning to-be-remembered items to familiar locations.

The two traditions do have important differences. Memory palaces exist solely in the imagination; hand mnemonics exist half in the mind and half in the flesh. Another difference lies in their intended use. Memory palaces are idiosyncratic in nature, tailored to the quirks of personal experience and association, and used for private purposes; they are very much the province of an individual. Hand mnemonics, by contrast, are the province of a community, a tool for collective understanding. They offer a way of fixing and transmitting a shared system of knowledge. They serve private purposes, certainly — such as contemplation, in the case of the Mogao mnemonic, or calculation, in the case of Bede’s computus. But they also have powerful social functions in teaching, ritual, and communication.

The richness of this overlooked tradition is glimpsed through its ubiquity. In medieval Europe, Christian hand mnemonics were commonplace. Several echo the Mogao system in appending key teachings to manual loci. A 1466 woodcut from Germany, titled The Hand as the Mirror of Salvation, assigns a different spiritual stage to each finger: God’s will to the thumb; examination to the index; repentance goes on the middle; confession is pinned to the ring; and the pinkie gets satisfaction.⁷ A 1491 devotional treatise, also from Germany, offers readers a “digital” table of contents: the book’s one hundred meditations are distributed across the hands.⁸ Another illustration in the same work populates the hands with miniature portraits of key Christian figures: apostles and saints gaze out from each of the twelve major divisions of the four fingers; Mary and Jesus share the thumb.⁹

In different times and places, hands also furnished mnemonic maps of sound. The so-called “Guidonian hand” owes its name to the eleventh-century Italian music teacher and scholar, Guido d'Arezzo. Arranging the different pitches in a scale onto the joints, he developed this technique to help students learn “unheard melody most easily and correctly”.¹⁰ Oddly, Guido’s own writings never depict the hands explicitly, but history nonetheless credits him, and, for centuries after his death, the Guidonian hand was a mainstay of musical instruction. One scholar has described it as “fundamental conceptual equipment” for all musicians of the time.¹¹

Other thinkers in Europe — perhaps inspired by Guido — developed systems for learning the sounds of language. In the 1400s, the writer John Holt devised a hand-based technique for remembering the declensions of Latin, and, in 1511, the German scholar Thomas Murner proposed a hand mnemonic for parsing German speech.¹² These authors were a few centuries behind their counterparts in China, however, where the hand had long figured in phonology. As early as the thirteenth century, Chinese scholars were projecting syllable charts — often called “rime tables” — onto the palms and fingers. A version from the 1600s maps thirty-two key sounds across the fingers, sixteen to each hand.¹³

In Europe, a number of mnemonics, sprung from the rootstock of Bede’s system, used the hand to reckon time. A remarkable example comes from a 1582 volume of practical astronomy by Jehan Tabourot, a French polymath best known for his work on dance, who published under an anagrammatic pen name, Thoinot Arbeau. The volume is a slim sixty-one pages, but eleven of those pages include images of hands — presented in various configurations and layered over with different kinds of data. Among these is a mnemonic for keeping track of a notorious calendrical quirk: the alternation of long and short months. The image shows a left hand; the thumb, middle, and pinkie fingers are extended, while the index and ring fingers are curled back toward the palm. The system begins with March, pegged to the extended thumb (31 days); then proceeds rightward to April on the curled-in index finger (30 days); then to May on the extended middle finger (31 days); and so on. It continues by traversing the five fingers twice, ending with January (31 days) on the thumb and February (28 days) on the index.

One of the most ambitious of all hand mnemonics was not tailored for time or sound or any one type of information. It was presented by Girolamo Marafioti of Calabria in a 1602 treatise on the arts of memory.¹⁴ The system consists of a map of ninety-two manual loci — twenty-three on the front and back of each hand — each housing a different geometric symbol: a crescent moon, a chalice, a circle with horns, what looks like a lemon. To use the system, one simply assigns a to-be-remembered tidbit to each locus. One might, as Marafioti suggests, use it to remember a group of people arranged by status, age, or other characteristics. The system compresses the features of a memory palace — the use of familiar terrain and distinctive images, its customizability — into a handy pocket-sized device.

A global survey of hand-mnemonics includes Jewish hand-calendars that resemble Bede’s computus¹⁵; the hand-based techniques with which mariners tracked moon and tide¹⁶; an elaborate manual system for remembering key moments in Dutch history; the mnemonic alphabet from 1579 in which different hand shapes represent different letters¹⁷; and a variegated vein of Chinese medical mnemonics.¹⁸ A truly comprehensive treatment would also scout the borderlands of this tradition. In some cases, the hand is mentally inscribed with information, but the primary function does not seem to be as a memory aid. Take, for example, alphabets used for communication with the deaf that relied on manual loci¹⁹; hand-charts studied by practitioners of chiromancy and the Kabbalah²⁰; systems for converting the hand into a sundial; and bodily maps used for divination and exorcism. In the last case, for instance, a Chinese illustration from 1152 invites readers to press on various parts of the hand (called mu or “eyes”) to dispel different kinds of evil.²¹

Spending time amid this rich tradition, questions arise. First, what makes the hand so popular as a mnemonic prop? A large part of the answer, surely, involves portability. The hands are always, well, ready to hand. Another part is familiarity. Though popular wisdom stresses how well we know the back of our hand, the palm is hardly terra incognita. A further advantage stems from how hand mnemonics offer both visual and kinesthetic routes to memory: they are both seen and felt.²² A final part of the answer is that the human hand can be parsed and construed variously. Seen one way, we have a perfect housing for ten virtues; seen another, we have a fitting framework for twelve apostles, thirty-two syllables, or a hundred meditations.

But why did hand mnemonics emerge when they did? What niche did they fill? The examples considered here suggest the tradition flourished in a period when literate and oral cultures coexisted, a time when some — the scholarly elites — were developing complex systems of knowledge in monasteries and universities, while others — the broader public — were trying to master those systems and use them in everyday life. Hand mnemonics may have been perfectly positioned to shuttle between these two cultures. They bridged the voice and the pen, offering, to the trained imagination, a kind of living inscription.

The thorniness here is that it’s difficult to say with confidence when exactly hand mnemonics flourished. We could assume they got their start around 1200 years ago, based on the earliest surviving examples. But it’s quite plausible that similar techniques had been in use for centuries or longer. Perhaps no earlier evidence survives because hand mnemonics were so widely used, so mundane, that no one thought to mention them. Recall that Bede didn’t bother to illustrate his famed system. Neither did Guido.

It's also hard to determine when and why hand mnemonics faded out — that is, if they have. Many continue to remember the alternation of long and short months by projecting them onto the knuckles, an update of Tabourot’s system.²³ Japanese students, channeling Bede, sometimes employ a hand-calendar for determining the day of the week on which a given date will fall.²⁴ In the US, hand-based maps — that is, configurations of the hand that resemble a particular geography — are used by residents of Michigan, West Virginia, Alaska, and other places.²⁵ Hand mnemonics are still used to teach the “right-hand rule” in physics classrooms²⁶ and remain especially popular in medicine, with more introduced all the time. Teams of doctors recently proposed manual systems for remembering the expected values of certain diagnostic tests, the anatomy of the brachial plexus and the lungs.²⁷ We increasingly stash our thoughts in virtual realms, but we sometimes still reach for that primordial “digital” repository in our pockets.