Writing styles used on early Vietnamese coins often had meaning and purpose. Some of  this purpose can be understood from  looking at Chinese coins that may have influenced Vietnamese coins of the time. The most widely circulated Chinese coin in early times was the Tang Dynasty Kai Yuan Tong Pao, whose style can be seen in several Vietnamese coins from the early Dinh dynasty to the late Tran dynasty.  A coin from the Ly dynasty, Thien Tu Nguyen Bao, is key to understanding how and why the Vietnamese used the Kai Yuan style, and what it symbolized. It appears that the Kai Yuan style also relates to the use of reverse characters on Vietnamese coins!

The first coins from the early Dinh and Anterior Le dynasties had rather irregular calligraphy but many like examples B and C used a Bao copied from the Chinese Kai Yuan coin (A). Thien Tu type I (D) uses a Kai Yuan style Bao, has a left hook Nguyen like the Kai Yuan , and the Thien is also of early Chinese style. Several coins from the later Tran dynasty also use the Kai Yuan Style, (E F G H K).

These Kai Yuan style coins include all early Vietnamese coins with characters on the reverse. This is not accidental and must have been a rule. Coins with the character Nhat meaning "one" and Nguyen meaning "beginning" were probably issued in the first year of the reign. The Thieu Phong with reverse "15" marks the 15th and last year of the reign. A reverse moon would have similar meaning. This rule probably derives from the meaning of Kai Yuan, Kai means portal or entrance and Yuan mean new or beginning. The Tang dynasty Kai Yuan is one of the most important Chinese coins ever issued and often used reverse moons and reverse mint mark characters.

Toda's early work on Vietnamese coins, http://art-hanoi.com/toda, was pioneering in the west but it is now 130 years old and some of its information is now known to be inaccurate. Many diminutive coins that Toda assigned to early emperors are now known to be later private mint coins. Other coins are reported with the four characters in a different order from known examples. Toda's images are all hand drawings and many use a single general purpose style that could not be on all the different coins.

However, Toda does report a Thien Tu coin for the Ly dynasty, however it is Thien Tu Thong Bao, which is doubtful as all Ly dynasty coins use the characters Nguyen Bao.

Several of Toda images of the Ly dynasty, http://art-hanoi.com/toda/12.html, use the same general purpose writing style and some have the characters in the wrong order or use the character Thong in place of Nguyen, such as Thien Cam Thong Bao, which should read Thien Cam Ngyuen Bao.