Document preferences for historical abbreviations

(Problems reported by Michael Deckers.) * NEWS, Theory: Document this.
eggert · Jan 16, 2017 · 7c9253c · 7c9253c
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diff --git a/NEWS b/NEWS
@@ -80,6 +80,9 @@ Unreleased, experimental changes
 
   Changes to documentation and commentary
 
+    The 'Theory' file now better documents choice of historical time
+    zone abbreviations.  (Problems reported by Michael Deckers.)
+
     tz-link.htm now covers leap smearing, which is popular in clouds.
 
 

diff --git a/Theory b/Theory
@@ -209,6 +209,19 @@ in decreasing order of importance:
 	If there is no common English abbreviation, use numeric offsets like
 		-05 and +0830 that are generated by zic's %z notation.
 
+	Use current abbreviations for older timestamps to avoid confusion.
+		For example, in 1910 a common English abbreviation for UT +01
+		in central Europe was 'MEZ' (short for both "Middle European
+		Zone" and for "Mitteleuropäische Zeit" in German).  Nowadays
+		'CET' ("Central European Time") is more common in English, and
+		the database uses 'CET' even for circa-1910 timestamps as this
+		is less confusing for modern users and avoids the need for
+		determining when 'CET' supplanted 'MEZ' in common usage.
+
+	Use a consistent style in a zone's history.  For example, if a zone's
+		history tends to use numeric abbreviations and a particular
+		entry could go either way, use a numeric abbreviation.
+
     [The remaining guidelines predate the introduction of %z.
     They are problematic as they mean tz data entries invent
     notation rather than record it.  These guidelines are now