Thanks for your attention, I am a biginner of Mathematica. I am confused by the command **@** and **@@**. From the Help I learnt **@@** has the function of *apply*. However, if I input **Cos@@5** the output is 5, while if I input **Cos@5**, I got **Cos[5]**. By this it seems **@** rather than **@@** has the function of apply. 

However, when I tried the function *Plus*, **Plus@@{1,2}** gave me **3** as wanted while **Plus@{1,2}** just gave me **{1,2}**. Could anyone help me with the difference (or relation) between **@** and **@@**?