STRICTLY, ABBREVIATION covers both initials, as in A.D., B.C., a.m., p.m., and contractions, as in Gen. (Genesis), sha'n't, and bldg. Whereas M.L. is initials, Med. Lat. is a contraction; and Med. L. is a mixture.
Contractions fall into three groups: Med., Medieval; schol., scholarship; Gen., Genesis; abbr., abbreviation; and other such formations, where the point usually falls at the end of the first syllable of a word;
don't and sha'n't (often written shan't) form, like isn't, contractions that are words in their own right; c'd, sh'd, b'l'd'g, rec'd, for could, should, building, received, are preferable to the more usual cd., shd., bldg., recd., but, in practice, they are inferior to cd, shd, bldg, recd.
Dr., Ld., Mr., Mrs., St., are much inferior to Dr, Ld, Mr, Mrs, St, because the respective r, d, r, s, t, form the final letter of Doctor, Lord, Mister, Mistress, Saint-why, in the name of sense, insert a period where none is needed? Compare, in addresses, the conventional Rd. for R'd (never used): the sensible contraction is Rd, without a period. The same good sense might well be applied to contracted given-names: why write Wm. for Wm, Jas. for Jas (James)?
In chemistry, physics, electricity and several other sciences, it is customary, whether in contractions or in initials, to omit points: thus: na or Na, natrium, and amp-hr or Ah, ampere hour, and cm, not c.m., centimetre, H, hydrogen, and O, oxygen.
For non-scientific and non-technological terms, it is usual to point the initials, as in P.M., Prime Minister-p.m., post meridiem (after 12 noon)-A.D., anno domini- C.W.S., Co-operative Wholesale Society-W.E.Gladstone, William Ewart Gladstone. Personal initials are always pointed, as in J. and J.B.: and it seems advisable that they should continue to be so.
But from the United States of America has come a practice that is rapidly growing and that could advantageously become universal. If it did, it would merely fall in line with the very general abandonment of points in chemistry, physics, electricity, etc.