Which and That: The Proper Usage in Grammar

     For twenty-seven years, I always thought “which” and “that” were interchangeable. I thought you can just randomly use either one in every sentence you write until recently, I saw an article online explaining that they are not the same.

     Take a look at these examples:

1. The pen, which has blue ink, is mine.

2. The pen, that has blue ink, is mine.

     Sounds like they are the same right? Well, not very much.

     The first sentence suggests that I only have one pen and the clause “which has blue ink” only adds detail about my pen. If you remove the clause “which has blue ink” from the sentence, it doesn’t affect the meaning of the sentence and the pen would still be mine. 

     The second sentence suggests that there are many pens but the one that has blue ink is mine. You cannot remove the clause “that has blue ink” because it makes my pen different among other pens.

     In other words, use “which” if the clause is unnecessary and does not affect the meaning of the sentence. Use “that” if the clause affects the meaning of the sentence.

     Still confused? Let us simplify things from here on and take a look at these examples:

1. Gold, which has chemical symbol AU, is used to make jewelry.

2. The necklace, that has a pendant, is made of gold.

     The first sentence used “which” because only gold has the chemical symbol AU in the periodic table of elements while the second sentence used “that” because not all necklaces have a pendant.