A linotype matrix
On the linotype system a matrix would hold the information of the types face. After the casting of the line o’ type the matrix would be guided onto a rail and drop down into the appropriate compartment where the keys would fit. And it would wait there for its next casting. See the wonderful Linotype film for more information.
‘Projections on the inside of the triangular opening at the top of the matrix. There are seven of these teeth on either side. The teeth which are left in are called the tooth combination. As the matrix travels along the distributor bar, it is supported by corresponding teeth on the bar. At a predetermined point, the teeth on the bar are cut away. The matrix, being no longer supported, drops through the channel entrance and to its proper place in the magazine. A matrix with all the teeth in is called a "pi" matrix, and passes all the combinations on the distributor bar, falling down the pi chute to the pi stacker.’
‘Useful Matrix Information‘ – October, 1937
The on/off binary activation from outer- to innermost teeth defines the order of the characters. The order follows the most frequent letters in the english language – the twelft first characters being: etaoin shrdlu.