BK.

The following table is compiled from various sources, across various languages. When English abbreviations or acronyms come from a non-English source, this is noted.
Entry	Source	Expression	Field
bk.	English	Black	Physics
Source: compiled by the editor, based on several corpora (additional references).

Commercial Usage: BK.

Domain	Title
Books	Sin Killer (McMurtry, Larry. Berrybender Narratives, Bk. 1.) [UNABRIDGED] (reference) Olivia's Touch (Stoks, Peggy. Abounding Love, Bk. 1.) (reference) O Come All Ye Faithful: Hymns of Adoration and Joy to Celebrate His Birth (Great Hymns of Our Faith, Bk. 2) (reference) The Resurrection File (Parshall, Craig, Chambers of Justice, Bk. 1.) (reference) All Together in One Place: A Novel of Kinship, Courage, and Faith (Kirkpatrick, Jane. Kinship and Courage Historical Series, Bk. 1.) (reference) (more book examples)
Source: compiled by the editor from various references; see credits.

Photo Album: BK.

Thumbnail	Description & Credit
	W. on Main St. from cor. of St. Paul, Dallas, Texas--Praetorian, Kirby, Exchange Bk., and Republic Buildings. Credit: Library of Congress.
Source: pictures compiled by the editor from various references; see picture credits.

Alternative Orthography: BK.

Hexadecimal (or equivalents, 770AD-1900s) (references)

Leonardo da Vinci (1452-1519; backwards) (references)

Binary Code (1918-1938, probably earlier) (references)

HTML Code (1990) (references)

ISO 10646 (1991-1993) (references)

Encryption (beginner's substitution cypher): (references)

Abbreviations & Acronyms: BK.