The "muntin" is the upright between the two stiles.    From Wordnik.com.  [Handwork in Wood] Reference

The bead moulding of the skirting is only partly removed, as indicated, leaving a solid portion to which the muntin is skew-nailed.    From Wordnik.com.  [Woodwork Joints How they are Set Out, How Made and Where Used.] Reference

It is used in knock-down furniture and in connecting a muntin to a rail, where it is desired that the muntin should run thru and also that the rail be continuous.    From Wordnik.com.  [Handwork in Wood] Reference

The interior teems with antiquity: ornate plasterwork, a huge inglenook, stained glass, elaborate panelling and a plank and muntin screen.    From Wordnik.com.  [Personal finance and money news, analysis and comment | guardian.co.uk] Reference