The letters I see can represent only a small number of the total sent to the lines.

Also, a letter or character which represents such a sound.

Each letter represents a single sound and there are no capital letters.

The letters also represent the number of people in each compartment at a particular time.

The top letter on many eye charts represents 20/200 vision.

The first letter represents the piece moved (see setup above).

Each letter represents a 2x2 block of numbers in the finished square.

As a whole, the letters represent a unified work of art the two brothers created together.

The first 2 letters represent the city the vehicle is registered in.

The letters in background represented the numbers zero to nine.