The back azimuth first, because it is the simpler idea and the one people still get wrong at three in the morning. $\alpha_b = \alpha \pm 180^{\circ}$, read aloud alpha-b equals alpha plus or minus one eighty. $\alpha$ is the forward azimuth — the bearing out, measured clockwise from north — and $\alpha_b$ is the same physical line sighted from the far end. Nothing has moved but the observer.

The discipline is the wrap. Add 180; if the result passes 360, take 360 off. Below 180 you may subtract instead and the two routes agree. So 035° becomes 215°, and 248° becomes 068° by way of 428°. The classic error is $360^{\circ} - \alpha$, which MIRRORS the line about north rather than reversing it — and on a resection that puts your fix on the wrong side of the sheet entirely.

Now the three norths. A compass needle does not point at the north pole and it does not point where your chart's grid says north is. Three different norths, two corrections between them, and one chain: $T = C + D + V$ — T equals C plus D plus V.

$C$ is the compass heading, what the card in front of the helmsman actually reads. $D$ is deviation: the error caused by iron and electrical gear in your own vessel. It is different on every heading, which is why it lives on a tabulated deviation card and not in your head, and a phone left on the binnacle can change it. $V$ is variation (magnetic declination): the angle between true north and magnetic north at the place you are standing. It belongs to the place and the year, drifts measurably, and comes off the chart's compass rose or the World Magnetic Model. $T$ is the true heading, the one a chart is drawn in. In this convention east is positive and west is negative, and going compass to true both corrections simply add.

The old rhyme — "east is least, west is best" — describes the OTHER direction, true to compass, and reversing it is the single most expensive sign slip in navigation. Get it backwards and you do not merely fail to remove the error: you apply it twice. On a 17° W variation that is a 34° mistake, held all watch. And the degrees on the end of your answer will not tell you which way you went — units guide, they do not confess. Fix the sign convention before the arithmetic, in writing.