In marking break indices, there may be cases in which the labeller is uncertain about the strength of the juncture. Specifically, it may be difficult to decide between two similar levels, such as 1 and 2. In such cases, the higher break index value should be chosen, and the diacritic ``--'' should be marked after it. Note that this ``--'' label does not mean that the strength of the juncture is somewhere in between BI 1 and BI 2, for example. It simply indicates that the labeller is uncertain --- (s)he is just not sure of the boundary strength.
Utterance 
yane
shows an example of labeller
uncertainty between BI 1 and BI 2. In this utterance, it is not clear
if the verb /tukema'su/ `attach' has been totally dephrased (accent
deleted) or if it is just very subordinate and thus realized in a
reduced pitch range (see also section 3.4 on accent
uncertainty). The break index 2-- is used here to indicate this
uncertainty about whether the adjacent words are grouped together into
a single unit, or remain separate. Since break index 2 often
corresponds to the boundary of an accentual phrase (unless there is
reason to suspect mismatch), the label 2-- indicates uncertainty about
the presence of an accentual phrase break. (Note that when the 2-- is
used, the labeller should also mark the appropriate accentual phrase
tones (H- and L% or wL%) in the tone tier.)
Example utterances 
narabu
and
nibanme
also show the use of 2--. In each
of these example utterances, the labeller is uncertain of the boundary
strength right before the final verb. Since utterance-final verbs are
often realized in a very reduced pitch range, it is ambiguous whether
the verb has joined together with the preceding words or forms a
separate unit. The break index 2-- marks labeller uncertainty due to
this ambiguity.
Utterance 
pinku_mado
shows an example of
labeller uncertainty between BI 2 and BI 3. Here, the labeller is
unsure of the boundary strength between /kono/ `this' and /pi'Nku/
`pink'. The rise in pitch on the word /pi'Nku/ indicates that it may
be the start of a new intonation phrase. However, the labeller is
uncertain that there is such a strong boundary, and chooses to mark
this break with 3--.
As with accent uncertainty described in section 3.4 above, there is no right or wrong answer in using the ``--'' uncertainty diacritic. It simply allows more freedom to the labeller to express her/his commitment to the break index value assigned. Therefore, labellers should not hesitate use this label liberally. It is only by flagging these uncertain areas that we will be able to go back and take a closer look at them in future research on phrasing.