The modern Greek alphabet contains only 24 distinct letters. But in lowercase there are 25 glyps: sigma is written ς at the end of a word and σ otherwise.
α β γ δ ε ζ η θ ι κ λ μ ν ξ ο π ρ σ ς τ υ φ χ ψ ω
Before the Greek alphabet was standardized for political reasons in the fifth century BC, several regional variants also had 25 letters, including glyphs such as Aeolian digamma (Ϝ, pronounced /w/) or Ionian sampi (ϡ, probably pronounced /ts/).
Α Β Γ Δ Ε Ζ Η Θ Ι Κ Λ Μ Ν Ξ Ο Π Ρ Σ Τ Υ Φ Χ Ψ Ω Ϝ/ϡ
The English alphabet has had 25 letters at some points in its history. J and V separated from I and U in the mid-1500s, but W was still considered a digraph (like TH) rather than its own letter.
A B C D E F G H I J K L M N O P Q R S T U V X Y Z
Several languages also use a Latin alphabet with fewer than 26 letters. In Classical times* Latin itself had only 23 (lacking J, V, W), and two of those (K, Y) appeared only in foreign words. The 26-letter convention comes from English, since early computer encodings such as 7-bit ASCII were designed only for English usage.
*excluding the reign of Claudius, because nobody liked his new letters