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.
When there are more than two lights in a window opening, the sashes must be subdivided and the panes of glass made smaller, and this subdivision is accomplished by means of pieces called "muntins" which are made so as to receive the glass in the same way as do the rails and stiles. In Fig. 319 is shown a window sash divided into lights, four in each sash, and at A is shown a muntin. In Fig. 320 is shown a full-size section through one of these muntins showing the way in which it holds the glass. A is the body of the muntin, BB is the glass on the two sides of it, held in place by the putty CC. The molding DD may be varied to suit the taste of the designer, but must be the same as on the rails and stiles.