Proficiency in a particular line
Gong sun Long , is a famous scholar lived in the State of Zhao during the Warring States Period (475-221BC). He had kept a circle skilled people around him. He often said, "A wise man should welcome anyone with a specialty."
One day, a man dressed in tattered and dirty clothes came to see him and said to him:" I have a special skill. "
Gong asked: "What is it?"
"I have a loud voice and I'm good at shouting."
then Gong turned to his followers and asked," Who is good at shouting?" But none of them answered "Yes ". So the scholar took the man in.
Some days later, Gong and his followers went on a trip. they came to a wide river and found the ferryboat was on the other side of the river. All of them had no idea. Suddenly, Gong thought of the shouting expert and turned to him, "Can you have a try?" The man realized it was the chance to show his skill. He shouted to the ferryman as loud as he could, "Hey, ferryman, come here, we want to cross the river." As his voice ended, the ferryman came to fetch them. Gong was very satisfied with the new follower.
Later, people use it to describe anyone who has a special professional skill.

一技之长
战国时期，赵国有位名士叫公孙龙。他手下聚集了许多有自己特长的门客。他常说："一个聪明人应该善于接纳每一个有自己特长的人。"
一天，有一个穿着很破烂的人来见他，并向他推荐自己："我有一项特别的本领。"公孙龙就问道："什么本领？"那个人回答道："我声音特别大，很善于叫喊。"公孙龙听了，就转身问他旁边的门客："你们有谁善于叫喊？"结果没有一个人回答"是"。于是，他就收下了这个很善于叫的人作他的随从。
没过多久，公孙龙和他的门客一起出外游玩。他们来到一条很宽的河边，发现渡船在河的另一头，所有人都不知怎么办好。突然，公孙龙想起他最近收的那个人的本领是善于叫，于是，他就转过头去对那人说："你大声叫对面的船夫，看能不能把他叫过来。"那人觉得展示他技能的时候到了，就尽力大声向对面喊：" 喂，船夫，过来，我们要过河。"当他叫声刚完，那对面的船夫就摇着船过来了。公孙龙对这个新收的门客非常满意。
后来，"一技之长"就用来形容一个人有一项特殊的本领。