What it means
Most Vedic charts use whole-sign houses: the sign holding the Lagna is the entire 1st house, the next sign is the entire 2nd, and every house is exactly thirty degrees. Placidus instead divides the time it takes a degree to rise, which produces twelve houses of unequal size whose boundaries - the cusps - usually fall partway through a sign.
How it is read
The consequence is the one that confuses people. A planet late in a sign can sit in the 4th house by whole-sign and the 5th by Placidus, from the same birth data. Neither chart is miscalculated; they answer 'which house' by different rules. Krishnamurti Paddhati builds on Placidus cusps deliberately, because its whole method turns on which sub-lord governs a cusp, and a cusp has to be a degree for that to mean anything.
What to watch for
At high latitudes Placidus degrades - the arcs it divides become extreme, and above roughly 66 degrees it fails to produce houses at all. That is not a concern for charts cast in India, but it is why the system is not universal, and it is worth knowing before treating a cuspal reading as more precise than a whole-sign one rather than merely different.
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