The copyright symbol is not necessary, but it is a good practice to affix the copyright symbol to works that copyright law governs. Attorneys who focus on copyright law call this “notice.” Notice is governed by 17 U.S.C. § 401. The statute says, “Whenever a work protected under this title is published in the United States or elsewhere by authority of the copyright owner, a notice of copyright as provided by this section may be placed on publicly distributed copies from which the work can be visually perceived, either directly or with the aid of a machine or device.”
The word may is key. This is because there is no requirement to use the copyright symbol.
However, should the copyright owner wish to use the copyright symbol, then these requirements exist:
(1) the symbol © (the letter C in a circle), or the word “Copyright”, or the abbreviation “Copr.”; and
(2) the year of first publication of the work; and
(3) the name of the owner of copyright in the work.
Should the copyright owner wish to use the copyright symbol, then “the notice shall be affixed to the copies in such manner and location as to give reasonable notice of the claim of copyright.”
Why should a copyright owner want to use the copyright symbol in order to put the notice of the copyright on the work? 17 U.S.C. § 401(d) states that defenses such as innocent infringement will not apply (in order to mitigate damages). Therefore, placing the copyright symbol on a work in order to give notice is an important step for a copyright owner to take in order to protect the copyright.