Students beginning the study of law almost immediately confront a congeries of Latin phrases, many of which end in the vowel "o".  For example, students will see in pari delicto (in equal fault) and ex delicto (from a wrong).  But they will also run into locus delicti (scene of the crime) and corpus delicti (body of the crime).  This change of endings may seem both mysterious and arbitrary, but it actually follows well-established rules of Latin grammar. 

The reason for the different endings is that most Latin nouns and adjectives have different endings depending upon how the word is used grammatically.  These are called case endings and they refer to the nominative, genitive, dative, accusative, ablative, and in some cases, locative and vocative cases.  In Latin, these case endings follow a very consistent pattern depending upon the declension of the particular word.  Fortunately, there are only five declensions and thus it isn't too difficult to memorize these patterns.

In the case of in pari delicto and ex delicto, the Latin word for crime or fault, delictum,  is used in a prepositional phrase (in meaning in, and ex meaning from).  These prepositions require the ablative case, which in the case of a second declension, neuter nouns such as delictum means changing the um to o.  In the case of locus delicti and corpus delicti,  the word delictum is a possessive and hence the case ending is i.  The table below lists the different endings of delictum: