▲a piece of baggage与an article of baggage均可表示“一件行李”，两者没什么区别。
《牛津高阶英汉双解词典》对piece的相关解释是：a single item of a particular type, especially one that forms part of a set（尤其指一套中的一件、一台等）。
《牛津高阶英汉双解词典》对article的相关解释是：a particular item or separate thing, especially one of a set（尤指一套中的一件物件或物品）。
▲注意，piece还有另外一此相关用法，此时与article用法不同：
1. amount of a substance (separated or broken from a larger piece) （从大物体上分离下来的）块, 片, 段, 截：
I stepped on a piece of glass, and my foot’s very sore. 我踩着一块玻璃，脚很疼。
Here is a piece of wood of the right size. 这有一块木头，其大小刚好适合。
She got a piece of bread stuck in her throat. 有一块面包卡在她的喉咙里。
2. amount or area of sth, esp for a particular purpose （某物质的）块, 片, 段, 截（尤指有某用途的）
a piece of land used for a particular purpose 特定用途的地
I put a piece of paper in the book to keep my place. 我在书里夹了一张纸以标示我读到什么地方。
The teacher wrote on the blackboard with a piece of chalk. 老师用一支粉笔在黑板上写字。
3. single instance or example of sth （事情的）项, 桩, 条
Let me give you a piece of advice. 让我给你一个忠告。
What a great piece of luck! 多幸运的事啊!
I have an amusing piece of news to tell you. 我有一条很有趣的消息告诉你。
This latest piece of information just confuses the issue. 最近的一条消息反而使问题更加复杂。
