Mesoamerican Long Count calendar the use of zero as a place-holder and presents one of the earliest uses of the zero concept in history. On Maya monuments, the Long Count syntax is more complex. The date sequence is given once, at the beginning of the inscription and opens with the so-called ISIG (Introductory Series Initial Glyph) which reads "tzik-a(h) hab’ [patron of Haab' month]" ("revered was the year-count with the patron [of the month]"). Next come the 5 digits of the Long Count, followed by the Calendar Round (tzolk'in and Haab') and supplementary series. The supplementary series is optional and contains lunar data, for
Fixation disparity A patient's associated phoria is the amount of prism needed to reduce their fixation disparity to zero minutes of arc. The Mallett Fixation Disparity Unit Instrument used to measure the associated heterophoria (or compensating prism). It consists of a small central fixation letter X surrounded by two letters O, one on each side of X, the three letters being seen binocularly, and two coloured polarized vertical bars in line with the centre of the X which are seen by each eye separately. The instrument can be swung through 90° to measure any vertical fixation disparity. The associated phoria is indicated