In 1920, census enumerators did not get much guidance from their handbook of instructions:
120. Column 10. Color or race.-Write "W" for white, "B" for black; "Mu" for mulatto; "In" for Indian; "Ch" for Chinese; "Jp" for Japanese; "Fil" for Filipino; "Hin" for Hindu; "Kor" for Korean. for all persons not falling within one of these classes, write "Ot" (for other), and write on the left-hand margin of the schedule the race of the person so indicated.
121. For census purposes the term "black" (B) includes all Negroes of full blood, while the term "mulatto" (Mu) includes all Negroes having some proportion of white blood.
By 1930, they got more words, but not much more guidance:
150. Column 12. Color or race.-Write "W" for white, "B" for black; "Mus" for mulatto; "In" for Indian; "Ch" for Chinese; "Jp" for Japanese; "Fil" for Filipino; "Hin" for Hindu; "Kor" for Korean. For a person of any other race, write the race in full.
151. Negroes.-A person of mixed white and Negro blood should be returned as a Negro, no matter how small the percentage of Negro blood. Both black and mulatto persons are to be returned as Negroes, without distinction. A person of mixed Indian and Negro blood should be returned a Negro, unless the Indian blood predominates and the status as an Indian is generally accepted in the community.
152. Indians.-A person of mixed white and Indian blood should be returned as Indian, except where the percentage of Indian blood is very small, or where he is regarded as a white person by those in the community where he lives. (Se par. 151 for mixed Indian and Negro.)
153. For a person reported as Indian in column 12, report is to be made in column 19 as to whether "full blood" or "mixed blood," and in column 20 the name of the tribe is to be reported. For Indians, columns 19 and 20 are thus to be used to indicate the degree of Indian blood and the tribe, instead of the birthplace of father and mother.
154. Mexicans.-Practically all Mexican laborers are of a racial mixture difficult to classify, though usually well recognized in the localities where they are found. In order to obtain separate figures for this racial group, it has been decided that all person born in Mexico, or having parents born in Mexico, who are not definitely white, Negro, Indian, Chinese, or Japanese, should be returned as Mexican ("Mex").
155. Other mixed races.-Any mixture of white and nonwhite should be reported according to the nonwhite parent. Mixtures of colored races should be reported according to the race of the father, except Negro-Indian (see par. 151).