The thumb is the digit the planetary scheme does not fit. The four fingers take their names from the mounts at their bases and carry them consistently across the whole literature, from the twelfth-century Latin chiromantia through Johannes ab Indagine's printed handbook of 1522 to the French system of the nineteenth century: Jupiter under the index, Saturn under the middle, Apollo under the ring, Mercury under the little finger. The thumb has no planetary finger-name of its own. What it has instead is the largest mount in the palm sitting at its base, the Mount of Venus, and the lower Mount of Mars just above it inside the curve of the life line, so that two planets are attached to one digit from below rather than one from above.
The tradition compensates by reading the thumb harder than it reads any finger. Cheiro treats it apart from the planetary scheme and divides it in two: the first phalanx, the one carrying the nail, is read for will, and the second for reason, with a long and firm thumb taken for resolution and a short or soft one for the want of it. Reading the shape of the hand and its digits in this way is chirognomy, the half of the subject that Casimir Stanislas d'Arpentigny set out in La Chirognomonie in 1843, as against chiromancy, the reading of the lines, which Adolphe Desbarrolles systematised in 1859. None of it has been shown to work. The associations are traditional and interpretive, and the accuracy readers report is well accounted for by cold reading.
Sources Introductiones apotelesmaticae elegantes in chyromantiam, physiognomiam, astrologiam naturalem, complexiones hominum, naturas planetarumJohannes ab Indagine (Johannes Rosenbach) with Palmistry for AllCheiro (William John Warner), La Chirognomonie, ou l'art de reconnaitre les tendances de l'intelligence d'apres les formes de la mainCasimir Stanislas d'Arpentigny, Les Mysteres de la MainAdolphe Desbarrolles