The first baths seem to have lacked a high degree of planning and were often unsightly assemblages of diverse structures. However, by the 1st century CE the baths became beautifully symmetrical and harmonious structures, often set in gardens and parks. Early baths were heated using braziers, but from the 1st century BCE more sophisticated heating systems were used such as under-floor (hypocaust) heating fuelled by wood-burning furnaces (prafurniae). This was not a new idea as Greek baths also employed such a system but, as was typical of the Romans, they took an idea and improved upon it for maximum efficiency. The huge fires from the furnaces sent warm air under the raised floor (suspensurae) which stood on narrow pillars (pilae) of solid stone, hollow cylinders, or polygonal or circular bricks. The floors were paved over with 60 cm square tiles (bipedales) which were then covered in decorative mosaics.