When a last letter in noun is a consonant, there is a tendency to plus '이' to noun. For example, 태형 is finished with ㅇ consonant, so we should add '이' to 태형 first, and add another postpositions. Therefore, 태형+형 -> 태형이 형. Indeed there are exceptions such as '태형씨', '태형아', '태형군' ...