Cartoonist Chris Schweizer has an informative overview of composition problems that can pop up when drawing. The Schweizer Guide to Spotting Tangents defines tangents as

...when two or more lines interact in a way that insinuates a relationship between them that the artist did not intend.

Chris follows with a delightfully illustrated list of ambiguities that often appear when composing pictures. They apply as much to photography as they do to drawing and cartooning and can often be overlooked when making an image. LINK: The Schweizer Guide to Spotting Tangents