Comitatives and instrumentals are morphemes, typically case affixes or adpositions, that express the semantic relations between participants of an event.  Comitatives show accompaniment, while instrumentals mark an NP as the instrument used by an agent to complete an action.  

<br><br>In languages like Finnish, which demonstrate <i>differentiation</i>, there are at least 2 distinct relator morphemes for comitatives and instrumentals, and neither can replace the other:

<br><br>a) Instrument: "Adessive <i>-llä</i>"
<br><i>Hän kirjoittaa kynä-<b>llä</b>.</i> 's/he write.3SG pen-<b>WITH</b>' for "S/he is writing <b>with</b> a pen."
<br>b)Accompaniment: "Comitative -<i>ine</i>"
<br><i>Läsnä oli V. V. vaimo-<b>ine</b>-en.</i> 'near be.PST.3SG V.V. wife-<b>WITH</b>-POSS.3' for "V.V. was present <b>with</b> his wife." 

<br><br>Glossing convention: Comitatives and Instrumentals will be  glossed WITH.

<br><br><b>Reference</b><br>
Stolz, Thomas & Stroh, Cornelia & Urdze, Aina. 2008. Comitatives and Instrumentals.
In Haspelmath et al (2008), chapter 52.
Available online at <a href="http://wals.info/feature/52">
http://wals.info/feature/52</a href>.
