Try a word
Type Vietnamese and see how every syllable is put together.
1The consonant on the left, the rhyme on the right
Every syllable is one square. The left half gives the first consonant, the right half the rhyme. The tone sits in a corner.
2One shape, one vowel
There are 22 shapes. On the right, each shape belongs to one vowel. A small stroke in one of five places tells how the rhyme ends.
Pick a vowel:
3The plain shape on the right is the vowel alone
The shape without a small stroke is a rhyme too: its vowel alone, with no final sound. Stroke 2 adds u or o to the vowel. The place decides: on the left it is a consonant, on the right a rhyme.
4The same shape twice is written once
When the consonant and the rhyme use the same shape, only the shape with the small stroke is written. It stands alone for the whole syllable. If the rhyme is the plain shape, both are written.
The whole table
Each row is one vowel with its shape: the plain shape and the five with a small stroke.
Where does the style guide come from?
The old manuscript of Quốc Âm Tân Tự explains its rhymes only with Nôm characters beside each shape, and not all of them can be read with certainty. A decoding is an attempt to read that table. Two are in use: the decoding qatt.org and the decoding omniglot.com. The page Decodings shows and compares them.
The style guide is not a decoding. It is the spelling that qatt.org has chosen for writing Vietnamese today, as a newspaper has a style guide of its own. It is built on the decoding qatt.org and tidies it up so that it can be learnt: every rhyme sits in the row of its vowel, the vowel alone is the plain shape, the vowel with u or o has stroke 2.
The characters that the style guide reads differently from the decoding qatt.org:
The places that became free stay empty, so a text written after the decoding is never read as a different word. The editor, the wiki and the Notes app also offer the two decodings in their menu.