_Leonardo's architectural studies fall naturally under two heads:_
_I. Those drawings and sketches, often accompanied by short remarks
and explanations, which may be regarded as designs for buildings or
monuments intended to be built. With these there are occasionally
explanatory texts._
_II. Theoretical investigations and treatises. A special interest
attaches to these because they discuss a variety of questions which
are of practical importance to this day. Leonardo's theory as to the
origin and progress of cracks in buildings is perhaps to be
considered as unique in its way in the literature of Architecture._
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