This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The girts are always made of the same width as the posts, being flush with the face of the post both outside and inside, and the depth is usually 8 inches, although sometimes a 6-inch timber may be used. The size is, therefore, usually 4 X 8 inches. A tenon at each end fits into the mortise cut in the post, and the whole is secured by means of a pin DD, as shown in Fig. 101. The pin should always be of hard wood and about 7/8 inch in diameter.
It is evident that if the girts in two adjoining walls were framed into the corner post at the same level, the tenons on the two girts would conflict with each other. For this reason the girts A which run parallel with the floor joists are raised above the girts B on which these joints rest, and are called "raised girts" to distinguish them from the others which are called "dropped girts." The floor joists are carried by the dropped girts, and the raised girts are so placed that they are just flush on top with the joists which are parallel to them.