Ten foot rule
The Ten Foot Rule refers to several rules of thumb in the SCA.
Heraldry: Conflict Checking
In heraldic conflict checking, it is a rule of thumb requiring that any arms clearly display visual difference to other devices from a minimum distance of ten feet. While a device can technically have enough difference to be clear of other devices, it can be ruled in conflict if it is judged that there is such a close visual similarity between it and another device, that most people can't tell the two apart from a distance. A ruling for visual conflict is a rare occurence.
From the SCA Rules for Submission:
"If the tinctures, shapes, or arrangement of the charges in a submission create an overwhelming visual resemblance to a piece of protected armory, the submission may be held to conflict even if sufficient theoretical difference can be counted between them.
A piece of armory is registered and protected, not the verbal description used to record that armory. The use of different terminology to describe two designs that are visually similar does not affect any potential for conflict that may exist. Thus, Or, a fess vert is not different from Vert, a chief and a base Or even though one could theoretically count sufficient difference between them from these blazons. Unusual cases may occur where contrast is weak and unusual arrangements of charges are employed, and in such circumstances the cumulative similarities between two pieces of armory may outweigh any specific differences. As an example, the cumulative effect of the similarities between Vert, ermined Or, on a mullet argent a lion rampant azure within a bordure embattled ermine and Vert, ermined Or, on an estoile argent a lion rampant azure within a bordure embattled erminois creates a strong possibility of confusion."
Visual conflict can be judged by holding up a standard-sized shield painted with the arms and comparing it from a distance of twelve feet (or, more commonly, a half-sized printout from the distance of five feet) from the next comparable device. If the differences aren't instantly clear, the arms might not be passed into the SCA armorial.
Heraldry: Device Recognition
A Ten Foot Rule (or six foot, it varies) is also used by heralds to determine whether a submitted device is recognizable or has major style problems. A color copy of the device is held up several feet from the crowd. If no one can tell what the charge or blazon is, than the device needs to be returned for re-drawing or to correct the problems causing the confusion.
Ten-foot and six-foot rules are also used in more authentic reenactment groups to determine the extent to which your kit can show evidence of modern construction, such as machine stitching or non-period fabric.