情場上有備胎，難道友誼的世界也有備胎？有的，這種備胎叫“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.
當你成為“備選項”，你就成了“架子上的朋友”。朋友們找人一起出去玩不會第一個想到你。當你的朋友開始約會，就不會再聯系你，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.
她總是有需要的時候才來找我，我是她的“備胎朋友”。

（英文來源：Urban Dictionary 編輯：丹妮）