The most common Dutch bracketed clock is the Friesland stoelklok; it is still produced according to a design which has hardly changed for over 200 years. Its development took place as much outside Friesland as in it, and was almost certainly introduced from the richer province of Groningen to the east. A small number from the early period clearly show both Groningen and Friesland characteristics, and it is difficult, sometimes impossible, to determine their origin. Some authorities hold that the Groningen stoelklok differs from the Friesland both in external design and in the movement. The case of the Groningen clock is generally stronger and more elaborately finished, greater attention being paid to the cast lead ornaments. The extra-thick back panel is often decorated with eels, mermaids or other fine carving. Examples from Friesland have mermaids simply in flat outline. Eels and mermaids, incidentally, were supposed to symbolize time because of their elusiveness. The construction of the verge movements of Groningen clocks was also more robust than Friesland examples; the pillars are usually plain or with a single decorative ring and a round or square plinth. The hands of Groningen clocks also differ slightly from those of Friesland models.