When do you use "middle" and when "center"?

They're synonyms, and are pretty much interchangeable.

Maybe, maybe, a native speaker would be more likely to say "middle" when speaking of things arranged along a line, i.e. one dimension, and "center" when speaking of two dimensional arrangements. Like we tend to talk about the middle of a line but the center of a circle. But it wouldn't be glaring to switch them.

• Middle means the half-position between two horizontal lines, levels, or a position in the Y-axis.
• Center means the half-position between two vertical lines, pillars, or a position in the X-axis.

In other words, middle gets typically used when there is a sense of progression between two points (either concrete or abstract). On the other hand, center gets used when the two extreme points are equally important or otherwise one no better than the other.

That's why there are center aligned texts and middle aligned images.

In sociology that's why there is a middle class, to distinguish from lower and upper classes. Similarly with the phrase mid-career because seniority is viewed as going upwards. These things are viewed as a ladder or otherwise something that goes upwards/downwards (i.e. vertically). There is no center class nor center career. Similarly for time progression, "I am in the middle of baking a cake."

Likewise there is the center of a stage and the center of attention. There isn't any middle stage nor middle attention, because those aren't thought of having "levels" but instead go along the horizontal axis.

Humans views things mostly two dimensionally, since both of our eyes faces the same direction and we don't have penetrative vision (e.g. sonar). Imagine that you're looking at a wall, what you're seeing is mostly two dimensions – there is the width of the wall which goes from left to right and the height of the wall which goes from bottom to top. You can't really see the depth of the wall since you don't have X-ray or sonar vision.

Because of this, we tend to think two dimensionally as well. For example career movement, there is a lateral movement in which you move to a different role with similar rank/salary/responsibility or a vertical movement in which you get promoted with improved salary & increased responsibility.

Reference:

• HTML Paragraph Alignment
• HTML Image alignment

Of course, the word "center" has some uses that do not overlap with "middle" at all, like in the names of organizations ("The Center for X").

There are also a number of fixed expressions where the words obviously cannot be interchanged, like "the middle of nowhere", "the middle ear", "the Middle East".

Differences that I think exist between "center" and "middle" in other contexts:

• The word "middle" is used more often than "center" when referring to time. You can say "the middle of the day" but we usually don't say "the center of the day". The phrase "in the middle of" is often used to refer to something that is unfinished and still in progress.

• The word "center" feels more appropriate to me than "middle" in certain contexts where you are talking about something that is "centered" in multiple dimensions, like the center of a circle: "the exact center of the bullseye" sounds better to me than "the exact middle of the bullseye".

I think my advice here is supported by the results of a Corpus of Contemporary American English (COCA) comparison that I looked at between "the center of" and "the middle of". The top collocates that were used with "the center of" more often than "the middle of" included things like "galaxy", "universe", "attention", "labyrinth", "planet", "cosmos"; the top collocates that were used with "the middle of" more often than "the center of" included "night", "afternoon", "decade", "July", "January", "sentence", "century", "divorce", "interview", "grade", "term", "concert", and "wilderness".

I'm a native English speaker. Intuitively, I think of "Middle" as "between things" and "Center" as in "in the position exactly 50% in one direction".

• The number between the two arrows to the left of my answer is in the "Middle" and it is also in the "Center". In this case I believe "Center" is a more specific term which is best to use here.

• The list of tags on this question is in the "Middle" of this questions content, as it is between the text and the share etc buttons, but it is not necessarily in the exact "Center" of the content.

• "Middle" can mean a general area, while "Center" is more precise. The letters L, M, N, and O are all in the "Middle" of the alphabet, but the "Center" of the alphabet does not really exist, since it would be in-between the letters M and N.

• With numbers, the "Middle" and "Center" of "1, 2, 3" is 2, the same for both.

"Middle" can be used in some contexts that "Center" cannot ever be used in. If you have one older sibling and one younger sibling, you are the middle child, but you are not the center child.

However, "Center" is not a subset of "Middle", they are different words. "Center" can also be used in some contexts that "Middle" cannot. As another user pointed out, the name of a building may be "The Center for X" but it can't be "The Middle for X".

In your context, I would have to ask, are the objects on the list equally sized, and are there always an odd number of them? If so, "Center" might be better, otherwise I would use "Middle".