1. Â Reference stations, found in Tide Table 1, have the times and heights of the high and low tides, listed by date.
2. Â Subordinate Stations, found in Tide Table 2, show the difference in times and heights for that station, based on the times and heights of the Reference Station.
3. Â To determine the tide level at times between high and low tide, you use the "Rule of Twelfths" Â Follow the AP form to apply this calculation. Â The form is found at:
The Reference Station is Boston. Â The Subordinate Station is Rainsford Isand. Â Boston is in Tide Table 1 - see page 68. Â Rainsford Isand is in Table 2, also on page 68.
First, look at the tides for Boston on 4/19/2004
 Time   Ht
 0456   -0.1
 1111   10.0
 1710   0.5
 2322   10.4
Next, look at the Subordinate Table for Rainsford Island
High tide is 0 hr and 00 min after Boston, (i.e., the same time). Â Its height is 0.95 times the height at Boston.Â
Low tide is 0 hr and 02 min after Boston, (i.e., 2 minutes after Boston low tide). Â Its height is also 0.95 times the height at Boston.Â
Finally, determine the height at Rainsford Island for the time in question, 1400 hrs.
1400 is nearly 3 hrs later than high tide.  From the  AP form, based on the rule of twelfths, the tide drops 50% of the range to the next low.
This results in the tide at 1400 being 5.0 above MLLW