The Placidus House System
This is the most popular house system in astrology and it was formerly called the Ptolemaic House System, though it is doubtful that Ptolemy who wrote in 3 AD used it. The “Ptolemaic” name was because Placidus de Titis, an Italian Monk & mathematician, made this after carefully reading and re-reading the Tetrabiblos while marking up the then popular Regiomontanus system. His changes were a break from his predecessors — he changed the emphasis from a space-oriented system to a time based emphasizing the how long it took for the ascendant to reach the meridian.
Part of its early success was that it kept popular features of the prior system — like the tenth house is always the Midheaven and it ties the ascendant to the first house, interceptions and unequal houses — while arguing for chart stability. This latter facet is also the Placidean’s greatest fault — in northern latitudes, like London, some houses can span 40 degrees while others to offset that shrink to eight. The argument against that issue is the shrinkage is happening in empty houses. Nonetheless, despite its handicaps it was used by London astrologers and their publications because it was the first “modern” house system and not French (see Morinus).
In America, its popularity grew because of Dalton’s Table of Houses while overseas in Britain, all of their 19th Century astrologers, Alan Leo, Sepharial, Charubel, the Golden Dawn & Aleister Crowley are among its fans. As Leo had a very popular magazine and put out his 1001 Nativities using it for reference, the Placidus format continued its stronghold into the 20th Century with Elbert Benjamine and the Church of Light, Marc Edmund Jones and his Sabian Assembly and Vivian Robson using that manual for their published works.
We take our example chart from the AFA archives. Harold was born in Baltimore, Maryland, but if we move his birthplace to different locations around the world always using the Placidus format, we can see the different mappings of his chart. This may not be problematic for everyone, but it is amusing.