When finding error in magnetic compasses, just remember that True Virgins Make Dull Company at Weddings, and that Dead Men Can't Vote Twice at Elections. Alright, so these are just memory devices, but you won't forget them, will you?
As for the first, the first letter of each word in the phrase True Virgins Make Dull Company at Weddings is representative of a different part of the formula used to find compass error. The diagram at right explains.
The second phrase is exactly the same as the first, but in reverse, and as such, rather than adding West error, you are now adding East error.

T	True compass bearing: the Zn value we found in prior steps
V	Variation locally: Found on charts or is given in problems.
M	Magnetic bearing: Sum of T and V. Is NOT the value found by PSC.
D	Deviation: Compass error based on heading and magnetic field of ship itself.
C	Compass Bearing: This is the bearing you found when you observed the body originally.
aW	Not a value, merely a reminder that you add West and subtract E error as you work down this form. Note: The difference between the T and C values is your total Compass Error.