It is (unofficially):

"Tan-To-Tan" = "Seam-To-Seam" + 4 inches

The "Tan" is the tangent point where the curvature of a formed head starts. The "Seam" is the point at which the head is welded to the shell; it is common for there to be a 2" straight length beyond the "Tan" for this purpose.

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Tan-tan is an abreviation for the length of a vessel from tangent line to tangent line. The tangent line is the point where the tank head is welded to the tank cylinder.

Tangent Line: Refers to the point of contact (tangency) between the cylinder and the knuckle portion of the vessel head. The distance from the tangent line on one head to the tangent line on the opposite head is known as the straight side, or tangent-to-tangent (T/T).

http://www.eng-tips.com/viewthread.cfm?qid=258699

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Note added at 2 hrs (2015-05-17 01:06:19 GMT)
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Tangent Line

A line that touches a curve at a point without crossing over. Formally, it is a line which intersects a differentiable curve at a point where the slope of the curve equals the slope of the line.

Note: A line tangent to a circle is perpendicular to the radius to the point of tangency.

http://www.mathwords.com/t/tangent_line.htm