a split-leaf psalter (sometimes known as a "dutch door" psalter) is a book of psalms in metrical form, in which each page is cut in half at the middle, so that the top half of the pages can be turned separately from the bottom half. the top half usually contains the tunes, and the bottom half contains the words. the tune and words can be matched by matching the meter; each meter is a specification of line length and (implicitly) stressed syllables; if a tune is in common meter, any set of "common meter" words will go with it (and vice versa).