How to Read the Eto Inscribed in a Sword's Mei — The Structure of the Jikkan Jūnishi and Cross-Referencing Era Names

When examining the nenki mei cut into the nakago (tang) of a nihontō, one sometimes notices two-character combinations such as "Hinoto-tori" or "Kanoe-ne" written beside the era name. These are the eto (sexagenary cycle). Era names are calendars established by rulers, while the eto is a counting system that cycles once every sixty years; in sword mei, these two have been used separately or together.

This article organizes the structure of the jikkan jūnishi from the perspective of reading sword mei, and examines in turn how to derive the eto from the Western calendar, common points of misreading, and how to handle era names of the Nanbokuchō period. The conventions of nenki mei themselves are covered in What Is a Nenki Mei, so this article focuses specifically on "how to read the characters used to count years."

The Combination of the Jikkan and Jūnishi

The eto consists of two characters pairing one of the jikkan (ten heavenly stems) with one of the jūnishi (twelve earthly branches). The jikkan are the ten characters Kinoe, Kinoto, Hinoe, Hinoto, Tsuchinoe, Tsuchinoto, Kanoe, Kanoto, Mizunoe, and Mizunoto; the jūnishi are the twelve characters Ne, Ushi, Tora, U, Tatsu, Mi, Uma, Hitsuji, Saru, Tori, Inu, and I; they advance one step at a time.

• Readings of the jikkan: Kinoe · Kinoto · Hinoe · Hinoto · Tsuchinoe · Tsuchinoto · Kanoe · Kanoto · Mizunoe · Mizunoto
• Readings of the jūnishi: Ne · Ushi · Tora · U · Tatsu · Mi · Uma · Hitsuji · Saru · Tori · Inu · I

Since the least common multiple of ten and twelve is sixty, there are sixty possible combinations and then the cycle repeats. Starting with Kinoe-ne, it proceeds through Kinoto-ushi, Hinoe-tora, and so on, ending with Mizunoto-i as the sixtieth, and then returns to Kinoe-ne. Since only pairings of odd-numbered with odd-numbered, or even-numbered with even-numbered, appear, combinations such as "Kinoe-ushi" or "Kinoto-ne" do not exist. If one comes across a non-existent pairing while reading a mei, it is likely that one of the characters has been misread.

Deriving the Eto from the Western Calendar

There are many occasions when, after converting the era name in a mei to a Western year, one wishes to verify the eto. Subtract 4 from the Western year, then divide by sixty and find the remainder. If this remainder is 0, the eto is Kinoe-ne; if 1, Kinoto-ushi; if 2, Hinoe-tora; and so on. The heavenly stem is determined by the remainder when dividing the remainder by 10, and the earthly branch by the remainder when dividing it by 12.

The following table shows examples of years verified by this calculation.

Genji is an era name that was itself changed in the year of Kinoe-ne, making it easy to remember that its first year falls on Kinoe-ne. Tsuchinoe-tatsu is familiar from the name of the Boshin War of Keiō 4 and Meiji 1. Keeping such well-known years as landmarks, when one sees a pair of characters in a mei one can immediately get a sense of "roughly what era this belongs to."

Watching Out for the Year Change in the Old Calendar

One must be careful that in the old calendar, the year did not change on January 1 of the Western calendar. The first month of the old calendar corresponds roughly to the period from late January to mid-February in the Western calendar. Therefore, when converting a sword whose mei reads "the 1st month" or "the 2nd month" to a Western year, the Western year may shift by one depending on the month.

Furthermore, in years when the era name changed, two era names overlap within the same year. For example, 1868 was Keiō 4, and the era changed to Meiji partway through the same year. The eto is Tsuchinoe-tatsu for both, unchanged. Even if the era names alone make a work appear to be from different periods, if the eto match it can be understood to be from the same year. This is why the eto, when one is uncertain about the reading of an era name, serves as a supporting clue.

Era Names and the Eto of the Nanbokuchō Period

During the Nanbokuchō period, the Northern Court and the Southern Court each used their own separate era names. Even for the same year, era names from the Northern Court and the Southern Court existed in parallel. This also shows up in sword mei — which era name was used is reflected in the inscription.

The national cultural property database includes jūyō bunkazai (Important Cultural Property) tantō bearing the Southern Court era name Shōhei. There is one inscribed "made by Sa Yasuyoshi" with "the day of the 2nd month of Shōhei 12" (National Register of Cultural Properties Database), and another inscribed "Bishū Osafune-jū Nagayoshi" with "the day of the 5th month of Shōhei 15" (National Register of Cultural Properties Database). On the other hand, the Agency for Cultural Affairs' Cultural Heritage Online includes a registered sword inscribed "Bishū Osafune Hidemitsu" with the era name and month "the 2nd month of Ōan 5" (Cultural Heritage Online). Ōan is a Northern Court era name.

When these mei era names are counted by eto as in the table above, Shōhei 12 corresponds to Hinoto-tori, Shōhei 15 to Kanoe-ne, and Ōan 5 to Mizunoe-ne. Even when the era names differ, the eto can serve as a common measure for comparing roughly which year a work belongs to within the same calendar. However, whether the eto themselves are actually cut into these mei has not been confirmed from the sources cited here. This is strictly a usage where the reader derives the eto from the era name and uses them for comparison.

Steps for Reading the Eto in a Mei

Here are the steps that are actually useful when reading the eto in a nenki mei.

1. Of the two characters, first confirm which character position is the heavenly stem and which is the earthly branch
2. Check that the odd-even relationship of the heavenly stem and earthly branch match (if they do not, it is a misread)
3. Derive the Western year from the era name, and check by the calculation above whether the eto matches
4. Check the old calendar month to confirm the Western year has not shifted by one
5. For works of the Nanbokuchō period, confirm whether the era name is from the Southern or Northern Court

Particularly worth noting are characters with similar shapes. The earthly branch Mi, the heavenly stem Tsuchinoto, and a third visually similar character are distinct as written characters, yet are difficult to tell apart in the small carved characters of a mei. The same is true of the earthly branch Inu, the heavenly stem Tsuchinoe, and a similar-looking character; the earthly branch Inu and the heavenly stem Tsuchinoe are very similar in form. However, as noted in the previous section, when the heavenly stem and earthly branch are paired, odd and even must match, so one can narrow it down using the other character as a clue.

The characters in a mei are carved into the nakago (tang) with a tagane (chisel), so the forms may differ from written characters. The technique of cutting mei is covered in The Craft of Cutting Mei, and where on the nakago mei are cut is summarized in Two-character Mei, Long Mei, and Mei Position.

Reading a Year Means Situating a Sword

The eto and era names are nothing more than small characters on the nakago, yet they greatly influence how a sword is situated. In schools where many generations of tōkō (swordsmiths) carried the same name, the presence of a nenki alone can narrow down which generation made the piece.

However, the reading of the nenki mei or eto alone does not determine the authenticity or value of a sword. The principle is to judge comprehensively from the sugata (shape), jigane (steel of the blade surface), hamon (temper line), and the condition of the nakago (tang); the reading of the nenki is just one of those clues. One verifies step by step that the era name and eto are not inconsistent, and that the work matches the style of the period.

When examining a sword, it is natural for the eye to be drawn to the hamon and sugata, but the calendar is also inscribed in those two small characters on the nakago. Knowing how to read them makes it possible to see more concretely what era a single sword has lived through.