Upper case letters mean capital letters, sometimes called majuscules. Upper case letters all have the same height.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Lower case letters mean small letters, sometimes called minuscules (from which comes the word “minus”). The height of lower case letters varies. Some are half as high as upper case letters, such as a, c, e, i, m, n, o, r, s, u, v, w, x and z. Some have ascenders (parts which stick up) such as b, d, f, h, k, l and t. Some have descenders (parts which hang down) such as g, j, p, q and y.
Years ago, when type was set by hand instead of by machine, a typesetter would take individual metal letters from a drawer or letter case. Capital letters were stored in one case and small letters were stored in another case along with punctuation and spacing markers. Capital letters were stored above small letters, leading capital letters to be called upper case letters and small letters to be called lower case letters.
Sometimes both capital and small letters were stored in a single case which could be set upright. When the case was organized, the capital letters were placed at the back of the case so that when it was set upright, capitals would be on top—hence, upper case.
The metal letters typesetters would see in the cases were reflections of the letters printed on the page. That means the metal letters faced the opposite direction from the printed letters. In a case, a “b” would look like a “d.” Words and sentences would be set in a way which to us looks backwards, but the printed version would appear as we see type today.
Capital letters go farther back in history than smaller letters which were introduced in about the ninth century. Smaller letters began as rounded, smaller versions of capitals. They were easier for scribes to write at a time when all writing was done by hand.
Today most English writing is done in lower case letters with capitals reserved for the beginnings of sentences and for proper nouns. Even so, capitals are often taught first to young children, perhaps because they are easier to distinguish. Capital B and capital D are easier to figure out than “b” and “d.”