One division of the Shashtiamsha spans 30 minutes of arc, half a degree, sixty to a sign and 720 in the zodiac. It is the finest division in the shodashavarga and it marks the practical limit of the whole technique.
The ascendant crosses half a degree in about two minutes. A birth time in error by five minutes therefore places the D-60 ascendant two or three divisions away from the truth, which is not a small distortion in a chart where each division carries its own name and character. Any D-60 reading that leans on its ascendant should state the recorded precision of the birth time alongside it.
The Moon crosses half a degree in roughly fifty five minutes, so even the Moon needs the hour to be right. The Sun takes about twelve hours and the slower bodies far longer, so their shashtiamshas are the stable part of the chart.
The counting rule itself is settled: the sixty divisions are counted from the sign being divided, and each carries a traditional name and nature rather than being read simply as a sign.
The Shashtiamsha enters at the dashavarga and continues in the shodashavarga, where the tradition gives it unusual weight for so fine a division. Its subject is the totality of past karma, which is offered as the reason a chart of this scale is thought to carry anything at all.