The parti-gyle, (the decision (parti) to split the brew (gyle) ) was/is literally used to make two or three beers of different strength from a single mash in different vessels, and was common practice until about 1725 when a new method of brewing was developed. It was called "entire" and meant that all the sugars from the mash were collected in a single vessel and boiled to create a beer of one strength. Most home brewers rely on "entire" brewing to create their beer and do not parti-gyle.
Sparge method brewers collect wort until they reach kettle / recipe / gravity volume, or the lowest accepted gravity, which is 1.010. The SOG (specific original gravity) is taken pre-boil - post-sparge and is the result of mixing what, in the days of yore, would have been the 1st, 2nd, and 3rd runnings, or the actual total sugar concentration in any given mash volume. Brew in a bag brewer's simply lift the bag and let the wort drain back into the kettle with the same result.
Out of one mash there could be three separate strengths of wort. The 1st runnings contain the highest percentage of sugars and a resultant higher ABV. To create the 2nd runnings, the mash was again flushed with water (sparged) and drained into a separate vessel than the 1st runnings. If there were sufficient sugars remaining, (based on volume) a third flush was initiated which resulted in a very weak (1.010 to 1.025) wort. Depending on the class served and region the wort might be mixed to elevate, or balance one of the runnings to the intended gravity, or depending on profit considerations, left to stand alone.
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