Letters can be distinguished strictly by physical representation in a number of different ways. Sometimes groups of letters are given in the form of a puzzle, and you have to figure out the rule by which they are grouped. Below are several different letter groupings, which illustrate how many letters in the alphabet possess some particular characteristic. Some of the groupings below assume a sans-serif font.

Letters	Rule
AEFHIKLMNTVWXYZ	Letters drawn with only straight lines
COS	Letters drawn with only curved lines
BDGJPQRU	Letters drawn with straight and curved lines.
BDO	Letters with no endpoints.
P	Letters with one endpoint.
ACGIJLMNQRSUVWZ	Letters with two endpoints.
EFTY	Letters with three endpoints.
HKX	Letters with four endpoints.
CEFGIJKLMNSTUVWXYZ	Letters with no enclosed areas.
ADOPQR	Letters with one enclosed area.
B	Letters with two enclosed areas.
BCDEHIKOX	Letters with horizontal symmetry.
AFGJLMNPQRSTUVWYZ	Letters without horizontal symmetry.
AHIMOTUVWXY	Letters with vertical symmetry.
BCDEFGJKLNPQRSZ	Letters without vertical symmetry.
HIOX	Letters with horizontal and vertical symmetry.
ABCDEFGJKLMNPQRSTUVWYZ	Letters without horizontal and vertical symmetry.
HINOSXZ	Letters with 180 degree rotational symmetry.
ABCDEFGJKLMPQRTUVWY	Letters without 180 degree rotational symmetry.
BCDGIJLMNOPRSUVWZ	Letters that can be drawn without taking pen off paper.
AEFHKQTXY	Letters that can't be drawn without taking pen off paper.
COPSUVWXZ	Capital letters that look like the corresponding lower case letters.
ABDEFGHIJKLMNQRTY	Capital letters that don't look like the corresponding lower case letters.
IJ	Letters that are dotted in lower case.
ABCDEFGHKLMNOPQRSTUVWXYZ	Letters that aren't dotted in lower case.
BDFHKLT	Letters that have ascenders in lower case.
ACEGIJMNOPQRSUVWXYZ	Letters that have do not ascenders in lower case.
GJPQY	Letters that have descenders in lower case.
ABCDEFHIKLMNORSTUVWXZ	Letters that do not have descenders in lower case.
CDILMVX	Letters that are Roman Numerals.
ABEFGHJKNOPQRSTUWYZ	Letters that aren't Roman Numerals.
EHIS	Letters that consist of just dots in Morse Code.
MOT	Letters that consist of just dashes in Morse Code.
ABCDFGJKLNPQRUVWXYZ	Letters that consist of both dots and dashes in Morse Code.