U is for UV filters!
Back in the day, UV filters were an important part of the photographers arsenal...not so much anymore. Unlike the human eye, most of the older films were sensitive to the full spectrum of light. Ultra violet light could cause the film to fog so the UV filter was used to attenuate these frequencies. Modern films and digital sensors aren't as susceptible to this effect so UV protection isn't the priority it used to be. UV filters can still be used to protect the front element of the lens but be careful in choosing one...the uncoated variety can cause glare. Most modern filters have coatings to prevent that but not all of them...so, let the buyer beware.