When engines had to be designed with pressure ratios greater than about 5, to meet demands for reduced fuel consumption, a new stalling phenomenon came to light, rotating stall. It occurred at low compressor speeds and caused blades in the first stage to break. This troublesome speed area is known as "off-design" and required the invention of special devices to make the compressor work. The compressor worked well near its maximum speed, known as "design", with a fixed area convergence from entry to exit to go with the design values of compression/density and with fixed blade angles set to give low pressure losses. At low speeds the much lower compression didn't squeeze the air enough to get through the now too-small exit. The velocity triangle combined the now too-slow entry air with the blade speed and gave a stalling angle.