13.1b Contractions in abbreviations. [VI.27]

As a general rule contractions are used in abbreviations that represent a single word regardless of whether or not they could have been used in the complete word. Examples:

St. Paul, Minn.Little Rock, Ark.
Prof. Smithfirst ed. [edition]Ted Mead, Ed. [editor]
Phys. Ed. Dept.ch. 7, pg. 3

Note: When the abbreviation for chapter (ch) is followed by a period, the ch contraction is used. The contraction cannot be used if there is no period because it would be read as child.

EXCEPTIONS:
Do not use the in contraction in the abbreviation for inch (in) or inches (ins). [VIII.31.a] Examples:

8 ins.3ft, 6in8-in. dia.

Do not use contractions for two-letter postal abbreviations such as AR [Arkansas] and IN [Indiana].

Do not use the contractions for be, con, or dis in an abbreviation if they do not constitute the first syllable in the complete word. [XIII.43.a]
Examples: Belg. [Belgium] (Con)n. [Connecticut]

Do not use the contractions for be, con, or dis if they comprise the entire abbreviation.
Examples: Dis. [District] Be [beryllium] Con. [Consolidated]

Do not use a one-cell, whole-word contraction for an abbreviation.
Examples: Can [Canada] IT [Italy] do [ditto]