There is a difference between true north and magnetic north. When using a chart, this difference is called "variation". When using a map, it is called "declination". Most charts are aligned to true north. The compass points to magnetic north. Longitude lines run true north/south. In our area, the difference between true and magnetic north is between 12 and 14º. The variation changes yearly by a few seconds (60 seconds to a degree). As long as your chart is within a few years old this should not be a problem. Read the innermost circle of the compass rose to find the variation and the yearly change.
The adjustment you make for this difference depends on whether you are going from the chart to the compass or the compass to the chart:
Adjustment for variation must be made when using a chart. This is very important if there is a large variation or if accuracy is very important. On the east coast the variation is west and we must add.
Variation West - Magnetic Best
Variation East - Magnetic Least
Some compasses have an adjustment to correct for variation (the Silva Ranger does, but the Silva Polaris doesn't). We have found that the adjustment on the Silva Ranger doesn't hold up well to salt water.
Deviation is the error of the compass caused by metal or magnetic items close to the compass. Be careful to keep any items that could cause deviation away from the compass. Some hand pumps have metal inside. We have seen a knife on a PFD change a hand held compass reading.
Two opposite rules apply to variation depending on whether you are going from the compass to the chart or the chart to the compass.
Chart Bearing Rules
If you are working from the chart to the terrain.
Field Bearing Rules
If you are working from the terrain to the chart.