At present, inorganic glass or sapphire glass is mainly used for the windshield of the watch (the crystal-clear part protecting the dial), and the windshield is fixed to the plastic or metal case by joining them with adhesive or by inserting packing between the windshield and the case. Around the 1960s, when wristwatches, which were unaffordably expensive, started to become popular, a great many acrylic dome-shaped windshields with raised centers were used. At that time, there was no appropriate adhesion method for affixing a windshield to a case; when a slight force was applied to the domed acrylic and the diameter became slightly smaller, the windshield was put into the opening in the case, released and fixed. If the entire windshield was distended, the force to sustain the pressure that came to bear when the size was reduced was considerable. While it was true that it was more difficult to make flat surfaces - including the dial and back plate - decreasing the thickness of the outer periphery was effective for one of the important values in a watch – “thinness” - and a dome shape where the center was thicker and where this thickness decreased as you moved along toward the outer sections was convenient. This ordinary round watch follows a “watch-like” shape by adopting the shape of the time when watches started to become popular among people.