Non-alphabetic Symbols in the Tai Viet Script
There are five non-alphabetic symbols:

Two of the symbols, TAI VIET SYMBOL KON and TAI VIET SYMBOL NUENG, may be regarded as ligatures of the words meaning person and one, respectively. In the case of TAI VIET SYMBOL KON, the use or non-use of this symbol is used to distinguish between homophonous words.
Reference: Brase, Jim. “Proposal to encode the Tai Viet script in the UCS”, 2007 p. 9
This article formerly appeared on ScriptSource.