This book contains an investigation of the co-occurrence between the consonants in the triliteral and quadriliteral verbal roots of Arabic and Hebrew. The consonants are grouped on the basis of Manner or of Place. Both co-occurrence restrictions and co-occurrence preferences of consonants and of consonant groups are described in detail. The statistical test for pronomial proportions is used in order to determine the statistical significance of the results. These results are compared to those of earlier work by other authors on this subject. The findings are explained within the framework of generative phonology. The methods used are described in detail and the book contains a wealth of tabulated material which can be of great use to other investigators.