Letter series move through the alphabet by a rule. Convert letters to their positions (A = 1 … Z = 26) to see the rule clearly.
1Useful facts
Positions: A 1, E 5, J 10, O 15, T 20, Y 25 (remember EJOTY)
Opposite letters add to 27: A–Z, B–Y, C–X, … M–N
The alphabet wraps around: after Z comes A
2Patterns
Constant skips (+2, +3, …)
Growing skips (+1, +2, +3, …)
Two alternating series
Letter–number pairs that change together
Groups of letters that continue one long series