We don’t need to endlessly debate the “comma-before-and” issue. It all comes down to clarity.
If the comma is necessary to ensure that the meaning of the sentence is clear to the reader, then use it. If it is not necessary, you are not obligated to include it, but you also are not wrong to always use it if you prefer to.
This is called the serial comma issue because it involves a series, such as, We need to discuss A, B, C, and D. The key issue is whether there are a lot of words in the element that appears before and (in this case, C). Here is what you should remember:
- If there are only one to four words in the element before and, you probably can do without the comma.
- If there are five words or more, read it closely to see whether or not the reader will combine the last two elements in the series as one. A comma might be helpful.
- From a pure writing perspective, you are not wrong to always use the comma, which is what many journalistic publications do, just to avoid any risk of confusion.
If you need the comma in one sentence, and in the next sentence you have a series that does not require it, you don’t have to use it just be consistent. That’s all right. The point of punctuation is to help the reader, and if the reader will be fine without the comma, omit it.
Here is a sentence where the comma is necessary: We need to discuss the budget, the development of new products and a new startup company that we recently bought. Without a comma, it appears that we need to discuss the development of products and a new startup, but we didn’t develop a new startup, and the confused reader probably would pause. There are three things we need to discuss: the budget, product development, and a new startup company.
If you want to always include the serial comma because you were taught to do that, you are not wrong to use it. But if you are writing for a superior who never wants to see it, then unless you have another job lined up for tomorrow, you better think twice about putting it in.