Over time, variations on these basic tinctures were developed, particularly with respect to the furs. Authorities differ as to whether these variations should be considered separate tinctures, or merely varieties of existing ones. Two additional colours appeared, and were generally accepted by heraldic writers, although they remained scarce, and were eventually termed "stains", from the belief that they were used to signify some dishonour on the part of the bearer. The practice of depicting certain charges as they appear in nature, termed "proper", was established in the seventeenth century. Other colours have appeared occasionally since the eighteenth century, especially in continental heraldry, but their use is infrequent, and they have never been regarded as particularly heraldic, or numbered among the tinctures that form the basis of heraldic design.