Calculating import duties by the length of toy mouse's tail

Stitching a decorative design on the back pocket of a pair of jeans hikes the customs duty well above that paid for plainer pants. So when a United States clothing distributor recently decided to import a pile of designer dungarees, he simply had the Hong Kong supplier cut the pockets in two, then sew them together again using a fancy design stitch. The pants entered the US at the lower duty rate.

''Obviously the stitching was functional, since it held the pieces of pocket together,'' says Mary Wright, the Boston-based partner with Mandel & Grunfeld, the nation's largest law firm specializing in customs matters.

''It also happened to create a design that was very decorative.'' says Ms. Wright, who advised the importer on the stitching switch - a totally legal sidestep to the US tariff regulations.

According to Ms. Wright, this is just a sample of the sometimes bizarre business she deals in from her dark-paneled office in Boston's financial district. Her job, she says, is to interpret Byzantine customs regulations - helping clients snare the best deal.

Consider, for instance, the highly unpublicized wrangle over imported rodents - fake ones, that is: If the tail is short, the item is classified as a genuine toy - with a higher duty than those with long tails. Longer tails denote a fake rat designed for practical jokes; those pay a much lower duty.

Then there's the digital watch dispute. Since the electronic gadgets have no moving parts, customs lawyers argued they couldn't be classed as ''watch movements.'' They were eventually redefined as ''electrical articles not provided for elsewhere,'' saving importers as much as 50 cents on watches with a wholesale value of only $3.

Another case involved coats with removable sleeves. By importing the sleeves unattached, the rest of the coat came in as a vest - at a lower duty rate than if the coat were zipped together.

Other issues: Are tote bags shopping bags or handbags? They carry a lower duty as handbags. And what about frog's legs? They now come in as duty-free ''frog meat,'' but efforts to hike the tax are expected.