Dear Paul,
Yes, the Rules of Golf require in Appendix II that putters have a "lie" of "at least" 10 degrees off vertical. Here is the Rule:
http://www.usga.org/playing/rules/books/rules/appendix_II.html
The reason for the rule appears to be to ban croquet-like equipment (mallets) from golf to keep the games separate and to require golfers to master the traditional method of putting. See:
http://www.ruleshistory.com/clubs.html
This language first appears in Appendix II to the Rules of Golf 1976-1980:
The shaft or neck or socket of a putter may be fixed at any point in the head and need not remain in line with the heel. The axis of the shaft from the top to a point not more than 5 inches (127mm) above the sole shall diverge from the vertical by at least ten degrees in relation to the horizontal line determining length of head under Appendix IIa. The shaft in cross-section shall be generally circular or otherwise symmetrical.
Ref.:
http://www.ruleshistory.com/appendices.html
This is an "exception" to the normal rules for woods and irons that the shaft shall attach in line with the heel of the clubhead. The sense is to allow shafts to attach otherwise for putters but to make some effort to keep putters looking like the other clubs, i.e., having some "lie" that makes the stroke be side-on. Perhaps the "at least 10 degrees" is just someone's arbitrary pick.
The 1996 Appendix added that the shaft cannot tilt along the line of the putt off vertical by more than 20 degrees forward or backward.
Ref.:
http://www.ruleshistory.com/appendices.html#1996appI
My friend Jeff Jackson has a nice discussion of how the Rules affect putter design, but he doesn't go into the reason for the rule. Ref.:
http://www.swingweight.com/usga_rules.htm
Cheers!
Geoff Mangum
Putting Theorist and Instructor
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