情场上有备胎，难道友谊的世界也有备胎？有的，这种备胎叫“架子上的朋友”（shelf friend）。

You become a shelf friend when you become an option. You are not first pick of friends. When your friend starts dating someone and you are put up on a shelf until they have no other people to hang with. 
当你成为“备选项”，你就成了“架子上的朋友”。朋友们找人一起出去玩不会第一个想到你。当你的朋友开始约会，就不会再联系你，除非找不到其他人可以一起玩。

Example:

I was put up on the shelf until she had a need for me. I was her shelf friend. 
她总是有需要的时候才来找我，我是她的“备胎朋友”。