Stamp differences, the characteristics that determine which stamp is which, can be quantified into two types-the visually apparent and the equipment driven. Visually apparent differences-design difference, shades, perfs (or imperf) appeal to nearly all collectors whereas equipment driven differences such as watermarks and fluorescent varieties appeal to more limited numbers. US collectors are quite spoiled in this regard. US stamps were only watermarked for thirty years (out of 165 years of US stamps) and there are only two watermark types to recognize. But areas where watermark varieties are significant have always had a fall off in collector popularity as most collectors are not keen to the demands of smelly watermark fluid and trays. The second and third issues of Hungary, for instance, are distinguished by a very difficult to identify watermark differences. The watermarks on these stamps were a sheet wide design, rather than being repeated on each stamp, so each stamp has only a small portion of the watermark fall on it. And, to add to the difficulty, the two watermarks are broad line designs with few distinguishing differences between them. On many stamps not enough of the design differences show for a definitive determination of what stamp it is. Needless to say not many collectors have the mind set to get enthused over these issues.