Most of the major decorated initials have a basic geometry underpinning them. The Lindisfarne Gospels is a wonderful example of how a foundation of mathematical order can give rise to something beautiful and graceful.
When composing decorated initials, start from a simple geometric framework. The pages on Page Layout, and Signs and Symbols give some forms. Sometimes, the height of a decorated initial is equal to a particular number of lines of general text. The initial below is approximately 12 lines high.
This may have symbolic significance, or it may well be the case that if you've already got a framework of ruled lines, why not use that as your basis and save yourself some work? The method of measuring, pricking and blind ruling all of the lines on a page is very time consuming.
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