In writing rules of expressing sound changes, we have to know whether the sound change is unconditioned or conditioned. Unconditioned sound change happens when a sound experiences a particular change wherever that sound occurs in a language. However, only a few sound changes are completely unconditioned because there are restricted environments that the change does not take place or perhaps some other changes occur. An instance of completely unconditioned sound is the word *lani →lai (wind) in Motu language of Papua New Guinea. There has been unconditioned sound change of the sound *n*. Another example is the change the word *ndang→dang (no) in Bataknese language.
Conditioned sound changes or combinatory sound changes happen when a great many sound changes only take place in certain phonetics environments rather than in all environments in which the sound occurs. The environment is written following a single slash (/) if a change takes place only in a specific phonetic environment. The line (_) indicates the location of the changing sound with respect to the conditioning environment. The line is placed before the sound that condition change if a change takes place before some other sound; the line follows the conditioning sound if a change takes place after some other sound. For examples, [t] became [s] before front vowels in Motu language, [x] became [k] after [s] in Afrikaans language, and [p] became [v] between vowels in Banoni language. Besides, when a language experiences a whole series of sound changes, sometimes, it is possible to reconstruct the changes themselves and the order in which the changes took place.
To sum up, in writing rules of expressing sound change, we have to be aware in types of sound changes whether it is unconditioned or conditioned changes.
Crowley, Terry. 1997. An Introduction to Historical Linguistics. Oxford University Press: Australia.