Would like the forum to enlighten me on various aspects of a perpetual calendar clock along with meaning and valculation of equation of time, meaning and calculation of Mean time. I also request the forum to enlighten me on what is meant by NM, FQ, FM, LQ, NM on this clock and calculation. I know it might be a tall order to satisfy these queries on a forum but one can start from basics! Also, this is a similar clock that sold in Christies. It can be seen here A French large gilt-brass striking eight day four-glass mantel clock with perpetual calendar and moonphase , ACHILLE BROCOT, PARIS. CIRCA 1880 What do they mean by a 'four year indicator' in their description?