A picture of a linotype matrix is shown in Figure 72. It is a plate of brass formed with ears _A_ at the four corners. The type mold is shown at _B_ and of course the thickness of the matrix varies with the width of the type letter it is designed to cast. In the top of the matrix there is a V-shaped slot _C_ formed with teeth which are designed to engage grooves in a V-shaped bar over the matrix magazine. The teeth on the matrices are variously cut away and serve as keys to the different channels of the magazine. There is a different combination of teeth for each letter of the alphabet and all matrices of the same letter have the same combination of teeth. The teeth on the distributing bar are also cut away according to various combinations. After the line has been cast the matrices are all raised by their ears and fed one by one upon the distributing bar. Each matrix moves along until it arrives over the channel it is designed to enter when it comes to a gap in the teeth of the bar that are supporting it and drops into the magazine. Although there are but seven teeth on each side of the V-shaped bar, there are more than enough combinations possible to provide a characteristic one for every key of the keyboard. A matrix always has at least one pair of teeth in engagement with the bar until it reaches the gap that lets it fall into its own special channel of the magazine. The number of matrices of each character varies with the normal frequency of use of that letter. For instance, the letter _e_ is very commonly used, and there must be many more _e_ matrices than _z_ matrices, but in any case the number need not be very large because the process of casting a line and redistributing the type does not take very long and the matrices are soon back in the magazine ready to be used over again. The casting and distributing operations are entirely automatic and while they are in process the compositor is setting a new line. The method of spacing which was a later development of the linotype is also very ingenious. The compositor does not have to bother with the width of spaces required to fill out a line. He merely touches the space key at the end of each word and and a space bar drops into place. Each space bar is formed of a pair of wedges and after the line has been assembled the space bars are automatically expanded to fill out the line completely by mechanism which holds down one wedge member (A, Figure 73) and slides up the other (B). INDIVIDUAL TYPE CASTING AND COMPOSING [Illustration: FIG. 73.--A LINOTYPE SPACE BAR]