# Using the definite article describing a general thing [closed]

I have a question about using articles.

Consider the following sentence.

"The/a right side of a rectangle can be found ..." "The perimeter of a rectangle may be / is found by"

The question is : Should I use the definite or the indefinite article in this case ? I am describing not a particular rectangle, but any rectangle, for example this sentence could describe the formula to calculate the position of the right corner of a rectangle.

Please explain this rule, and also it is very important for me to know if I've made any mistakes in using articles in the question.

Thank you so much.

• Can you please explain more clearly. At least a full sentence is required for context, and preferably the couple of sentences preceding it. Do you mean 'right-hand', 'perpendicular', or 'correct' when you say 'right'? – Edwin Ashworth Mar 7 '17 at 12:16
• @EdwinAshworth thanks, in order to make it more clear, I've changed "corner" to "side" – ketazafor Mar 7 '17 at 12:20
• 'The right side of a rectangle can be found ...' is not an expression I ever used in many years of teaching maths. Again, do you mean 'appropriate' here (which needs prior context but would doubtless use 'the')? 'The perimeter of a rectangle may be / is found by ...' makes more sense. – Edwin Ashworth Mar 7 '17 at 12:25
• @EdwinAshworth thank you so much, I have added your expression to the question – ketazafor Mar 7 '17 at 12:34

Since you are specifying which part of the rectangle you are talking about, the 'right side of a rectangle', you should use 'the'. If it was an unspecified part of a rectangle you'd use 'a'.

compare:

A side (any side will do) of a (any) rectangle...

and:

The right side (specifically the right side) of a (any) rectangle...

• Thank you so much, so in general if we are referring to the particular part of an object we should use the definite article ? P.S. Please let me know, if I have made any mistakes in my question, it is really important for me. – ketazafor Mar 7 '17 at 12:33
• If you are specifying anything, use the definite article. For example, 'the fur of a cat', 'the bark on a tree', 'the windows of a house', etc (note, though, that you should say 'a part of an aeroplane' since the actual part isn't specified). If it doesn't matter what part of something you are talking about, use the 'a', eg 'a wing of an aeroplane' (it doesn't matter which), 'a hair of his head' (any hair will do) etc... – marcellothearcane Mar 7 '17 at 12:42
• Your question looks okay, but from a pedantic punctuation point of view, generally there is no space between the punctuation and the preceding word ('can be found...' rather than 'can be found ...', 'in this case?' rather than 'in this case ?' etc) but it's not really anything to worry about! – marcellothearcane Mar 7 '17 at 12:45