Your compass doesn't point at True North. It points at Magnetic North, and even that gets bent slightly by the ship's own iron and electronics. Three different "norths," three different course readings for the exact same direction over the ground:
Variation is geographic — the difference between True and Magnetic North at your position, printed on the chart. Deviation is local to your own vessel — the error your compass has at a given heading, from a deviation card.
Signed the way this site signs everything else — East positive, West negative — the whole chain reduces to one formula each way:
That's the same relationship the old rhyme "error East, compass least; error West, compass best" is getting at — an Easterly error makes the compass reading smaller than the true course, a Westerly error makes it larger. Going the other way, from Compass back to True, just reverse the signs: Magnetic = Compass + Deviation, True = Magnetic + Variation.
Variation isn't fixed — it drifts slowly year to year, which is why every chart states it for a specific date along with an annual rate: something like "14°E (2014), increasing 1' annually." To use it today, you have to age it forward.
Increasing is straightforward — the value just grows in the same direction already printed. Decreasing is where students get caught out: if the chart is old enough, the value doesn't just shrink, it can shrink through zero and come out the other hemisphere entirely. A chart reading 2°E, decreasing 15' a year, used 26 years after publication, isn't a small easterly value anymore — it's 4.5°W. Get the arithmetic right but keep the old label out of habit, and the answer is still wrong.