An acrostic (from the late Greek akróstichon, from ákros, "top", and stíchos, "verse") is a poem or other writing in an alphabetic script, in which the first letter, syllable or word of each line, paragraph or other recurring feature in the text spells out another message. A form of constrained writing, an acrostic can be used as a mnemonic device to aid memory retrieval. A famous acrostic was made on the Greek for the acclamation JESUS CHRIST, GOD'S SON, SAVIOUR which in Greek is: Iesous CHristos, THeou Uios, Soter (ch and th being each one letter in Greek and u is also y). The initials spell ICHTHUS same as ICHTHYS, Greek for fish; hence the frequent use of the fish by early Christians and up to now as a symbol for Jesus Christ.
The ease of detectability of an acrostic can depend on the intention of its creator. In some cases an author may desire an acrostic to have a better chance of being perceived by an observant reader, such as the acrostic contained in the Hypnerotomachia Poliphili (where the key capital letters are decorated with ornate embellishments), or as in the poem To Doctor Empiric (by Ben Jonson) which is a verse outlined after the word W-O-L-F giving emphasis to, and capitalizing the key letters so such acrostic is relatively easier to discern. However, acrostics may also be used as a form of steganography, where the author seeks to conceal the message rather than proclaim it. This might be achieved by making the key letters uniform in appearance with the surrounding text, or by aligning the words in such a way that the relationship between the key letters is less obvious. This is referred to as null ciphers in steganography, using the first letter of each word to form a hidden message in an otherwise innocuous text. Using letters to hide a message, as in acrostic ciphers, was popular during the Renaissance, and could employ various different methods of enciphering, such as selecting other letters than initials based on a repeating pattern (equidistant letter sequences), or even concealing the message by starting at the end of the text and working backwards.
The Dutch national anthem (The William) is an acrostic: the first letters of its fifteen stanzas spell WILLEM VAN NASSOV. This was one of the hereditary titles of William of Orange (William the Silent), who introduces himself in the poem to the Dutch people.
Here is a classic example of acrostic poem in English written by Edgar Allan Poe entitled simply An Acrostic:
Another example is from Lewis Carrol's "Through the Looking-Glass". The final chapter "A Boat, Beneath A Sunny Sky" is an acrostic of the real Alice's name: Alice Pleasance Liddell.
Children three that nestle near,
Eager eye and willing ear,
Pleased a simple tale to hear - Long has paled that sunny sky:
Echoes fade and memories die:
Autumn frosts have slain July. Still she haunts me, phantomwise,
Alice moving under skies
Never seen by waking eyes. Children yet, the tale to hear,
Eager eye and willing ear,
Lovingly shall nestle near. In a Wonderland they lie,
Dreaming as the days go by,
Dreaming as the summers die: Ever drifting down the stream -
Lingering in the golden gleam -
Life, what is it but a dream?
Children three that nestle near, Eager eye and willing ear, Pleased a simple tale to hear -
Long has paled that sunny sky: Echoes fade and memories die: Autumn frosts have slain July.
Still she haunts me, phantomwise, Alice moving under skies Never seen by waking eyes.
Children yet, the tale to hear, Eager eye and willing ear, Lovingly shall nestle near.
In a Wonderland they lie, Dreaming as the days go by, Dreaming as the summers die:
Ever drifting down the stream - Lingering in the golden gleam - Life, what is it but a dream?
Here is another example where the initial letters spell out the months of the year, entitled A Calendar Acrostic:
Other examples can be considerably more complex. The illustration shows a manuscript of the poem Behold, O God, in rivers of my tears by William Browne of Tavistock (found in Withington church, Gloucestershire), which is a triple acrostic showing the three crosses of Christ and the two thieves, each with a quotation from the New Testament account. On the central cross is the label "INRI" above the sentence "O God, my God, why hast thou forsaken me?"; on the outer crosses "If thou art the Christ, save thyself and us" and "Lord, remember me when thou comest into thy kingdom." The full text (with a few variants, but without the acrostics highlighted) can be found at page 4-5 of this link to Browne's works
There are some acrostics whose authenticities are disputed. For instance, the first Hebrew letter of each consecutive Hebrew name from Adam to the father of Abraham appears to form an acrostic that when translated in English reads: I WILL FORGIVE MY ENEMIES, HAVING COMPASSION, FORGIVING THOSE MADE FROM DUST A SECOND TIME. However, it is debatable whether this acrostic is the result of random chance or by design.
The acrostic appears to be highly structured. For example, the Hebrew word for forgiveness also means, "to lift up". Thus it may also read (and note the play on words), "I will lift up those who have risen up against me, having compassion, lifting up those (laid low in) the dust a second time."