When erect, a penis may be curved up (u-curved) or down (n-curved). In the picture, the penis on the left is curved down and the other on the right is curved up.
<a href="//commons.wikimedia.org/wiki/User:Erection" title="User:Erection">Erection</a>
CC BY 3.0
<span class="int-own-work" lang="en">Own work</span>