Block-laying starts with a level, properly marked foundation line, then blocks are set in a bed of mortar (a mix of cement, sand, and water) with each course checked constantly against a string line and spirit level to keep walls straight and plumb, since small errors compound quickly over multiple courses. Concrete mix ratios describe the proportion of cement to sand to aggregate by volume; a common general-purpose mix is roughly 1:2:4 (cement:sand:gravel), while a stronger mix such as 1:1.5:3 is used for structural elements like columns and slabs that bear heavier loads. Curing, keeping newly placed concrete moist, typically by watering it regularly for about 7 days and allowing a full 28 days before it bears significant load, allows the chemical reaction between cement and water to complete properly; concrete that dries out too fast cures weak and becomes prone to cracking. Skipping or shortening the curing period is one of the most common and costly mistakes on small-scale building sites.