For centuries, people have tried to explain how the sex of a child is determined, and it was not until scientists began to examine the nuclei of cells that they realised the chromosome sets in the nuclei of cells of men and women were slightly different. 23 out of 46 chromosomes in women's nucleus' are matched pairs, men on the other hand only have 22 matched pairs out of their 46 chromosomes, the 23rd consisting of two unmatched chromosomes. After much examination of the 23rd pair of chromosomes, in men the larger became known as the X-chromosome and the smaller the Y-chromosome. Females therefore have two X chromosomes, and males only one X and a Y. This 23rd pair of chromosomes both X and/ or Y are called sex chromosomes. The other 22 pairs of chromosomes (autosomes) are not involved in the sex determination.