Using a Bit Field

function IsPangram(const sentence : string) : boolean;
var
  letters : longword;
  c : char;
begin
  letters := 0;
  for c in sentence do
    if c in ['a'..'z'] then
      letters := letters or (1 shl (ord(c) - ord('a')))
    else if c in ['A'..'Z'] then
      letters := letters or (1 shl (ord(c) - ord('A')));
  result := letters = $3FFFFFF;
end;

A longword is a 32-bit unsigned integer. We use one bit per letter: bit 0 for a, bit 1 for b, …, bit 25 for z.

shl is Pascal's left-shift operator. 1 shl (ord(c) - ord('a')) produces a mask with exactly one bit set — the bit corresponding to the letter c. The or operator sets that bit in letters.

The in operator tests membership in a set literal: c in ['a'..'z'] returns true when c is a lowercase letter. Handling upper and lower case separately avoids a call to any case-conversion function.

$3FFFFFF is a hexadecimal literal equal to 67,108,863, which has its lowest 26 bits set — one for each letter of the alphabet. When letters = $3FFFFFF, every letter has been seen.

The for c in sentence loop iterates directly over the characters of the string without index arithmetic.