The question depends on what you consider to be English letters. The letters that this site uses are Roman letters.
Letters with both horizontal and vertical symmetry are: (lower case) - lox (upper case) - HIOX
,;'rtyuiop[][pouyfdfghjkl;'/.,mncvbnm,
No. DAY is a word typed in upper case letters. It is not a line - whether of symmetry or otherwise.
Lower case "o" and "x" both have two lines of symmetry. Upper case letters with two lines of symmetry are: "O," "X," "H" and "I."
The letters A H I O T X Y all have a vertical line of symmetry (as they look the same on the left as they do on the right).
O,I,H,andX * * * * * Not forgetting N, S and Z.
Lower case A doesn't have a line of symmetry. Upper case A has a vertical line of symmetry.
The answer depends of the font.In broad terms, in the upper case letters:F, G, J, L, P, Q and R.The others have reflective (or line) symmetry, or they have rotational symmetry, or both.
lower case i has 1 line of symmetry upper case I has 2 lines of symmetry
The letters: I O H X o x all exhibit bilateral symmetry and rotational symmetry, with O and X exhibiting it in both upper and lower case.
There are 10 letters in the alphabet that achieve rotational symmetry when displayed in their capital form. These letters: A, H, I, M, O, U, V, W, X, and Y.
Upper case letters are letters which are in capital format. You can define upper case letters by saying big letters. You can switch to upper case letters by holding shift and/or typing letters while caps lock key on. Lower case letters are letters which are not capital and can be defined by not applying Shift.