1.2.4 Combining Form
A combining form is the combination of a root with a combining vowel.In English，the most common combining vowel used in the formation of the combining form is the letter-o-，which links the root to the suffix or the root to another root.The root shown with a combining vowel is called“combining form”（Fig.1-2）.
Fig.1-2 Combining Form
When a suffix beginning with a consonant is added to a root，a vowel（usually an-o-）is inserted between the root and the suffix to aid in pronunciation（Fig.1-3）.Taking-logy for example，when the suffix-logy，meaning“study of”，is added to the root neur/o，meaning“nerve or nervous system”，a combining vowel is added between them：neur＋-o-＋-logy＝neurology（study of the nervous system）.
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Fig.1-3 A Combining Vowel Added Between a Root and a Suffix
Drop the combining vowel before a suffix that begins with a vowel.Taking-itis for example，when the suffix-itis is added to the root gastr/o（meaning“stomach”），the combining vowel-o-is omitted：
gastr/o＋-itis＝gastritis（inflammation of the stomach）.
Keep the combining vowel between two roots，even if the second root begins with a vowel，for example，when the root gastr/o is added to another root enter/o（meaning“intestine”），the combining vowel-o-between the two roots is kept：
gastr/o＋enter/o＋-logy＝gastr o enter o logy（the study of the stomach and intestine）.