International uniformity of braille alphabets

The goal of braille uniformity is to unify the braille alphabets of the world as much as possible, so that literacy in one braille alphabet readily transfers to another. Unification was first achieved by a convention of the "International Congress on Work for the Blind" in 1878, where it was decided to replace the mutually incompatible national conventions of the time with the French values of the basic Latin alphabet, both for languages that use Latin-based alphabets and, through their Latin equivalents, for languages that use other scripts. However, the unification did not address letters beyond these 26, leaving French and German Braille partially incompatible and as braille spread to new languages with new needs, national conventions again became disparate. A second round of unification was undertaken under the auspices of UNESCO in 1951, setting the foundation for international braille usage today.
Numerical order.
Braille arranged his characters in decades (groups of ten), and assigned the 25 letters of the French alphabet to them in order. The characters beyond the first 25 are the principal source of variation today.
In the first decade, only the top four dots are used; the two supplementary characters have dots only on the right. These patterns are repeated for the second decade, with the addition of a diacritic at dot 3; for the third, at dots 3 and 6; for the fourth, at 6; and for the fifth decade, by duplicating the first decade within the lower four dots.
Unification of 1878.
Braille is in its origin a numeric code. Louis Braille applied the characters in numerical order to the French alphabet in alphabetical order. As braille spread to other languages, the numeric order was retained and applied to the local script. Therefore, where the alphabetical order differed from that of French, the new braille alphabet would be incompatible with French Braille. For example, French was based on a 25-letter alphabet without a "w". When braille was adopted for English in the United States, the letters were applied directly to the English alphabet, so that braille letter of French "x" became English "w", French "y" became English "x", French "z" English "y", and French "ç" English "z". In the United Kingdom, however, French Braille was adopted without such reordering. Therefore, any English book published in braille needed to be typeset separately for the United States and the United Kingdom. Similarly, the letters for Egyptian Arabic Braille were assigned their forms based on their nearest French equivalents, so that for example Arabic "d" had the same braille letters as French "d". For Algerian Arabic Braille, however, the braille characters were assigned to the Arabic alphabet according to the Arabic alphabetical order, so that Algerian "d" was the same character as Egyptian "h". Thus an Arabic book published in Algeria was utterly unintelligible to blind Egyptians and vice versa.
In addition, in other alphabets braille characters were assigned to print letters according to frequency, so that the simplest letters would be the most frequent, making the writing of braille significantly more efficient. However, the letter frequencies of German were very different from those of English, so that frequency-based German braille alphabets were utterly alien to readers of frequency-based American Braille, as well as to numerically based German, English, and French Braille.
The 1878 congress, convening representatives from France, Britain, Germany, and Egypt, decided that the original French assignments should be the norm for those countries:
Gradually the various reordered and frequency-based alphabets fell out of use elsewhere as well.
This decision covered the basic letters of the French alphabet at the time; "w" had been appended with the extra letters, so the 26 letters of the Basic Latin alphabet are slightly out of numeric order:
For non-Latin scripts, correspondences are generally based, where possible, on their historical connections or phonetic/transcription values. For example, Greek γ "gamma" is written "g", as it is romanized, not "c", as it is ordered in the alphabet or as it is related historically to the Latin letter "c". Occasional assignments are made on other grounds, such as Greek ω "omega", which is written "w", as in beta code and internet chat alphabets, due to the graphic resemblance of Latin "w" and Greek "ω".
Basic correspondences.
Correspondences among the basic letters of representative modern braille alphabets include:
The 1878 congress only succeeded in unifying the basic Latin alphabet. The additional letters of the extended French braille alphabet, such as , are not included in the international standard. The French , for example, corresponds to print , whereas the in English and German braille transcribes , and the in Hungarian and Albanian braille is .
Alphabets limited to grade-1 braille.
Languages that in print are restricted to the letters of the basic Latin script are generally encoded in braille using just the 26 letters of grade-1 braille with their French/English values, and often a subset of those letters. Such languages include:
In these languages, print digraphs such as "ch" are written as digraphs in braille too.
Languages of the Philippines are augmented with the use of the accent point with "n", , for "ñ". These are Tagalog, Ilocano, Cebuano, Hiligaynon, and Bicol; "Ethnologue" reports a few others.
Languages of Zambia distinguish "ñ/ŋ/ng’" from "ng" with an apostrophe, as in Swahili Braille: "ng’" vs "ng". These are Lozi, Kaonde, Lunda, and Tonga. Ganda (Luganda) may be similar.
"Ethnologue" 17 reports braille use for Mòoré (in Burkina Faso), Rwanda, Rundi, Zarma (in Niger), and Luba-Sanga, but provides few details.
Congress of 1929.
In 1929 in Paris, the American Foundation for Overseas Blind sponsored a conference on harmonizing braille among languages which use the Latin script, which had diverged in the previous decades.
Congresses of 1950–1951.
When additional letters are needed for a new braille alphabet, several remedies are used. 
A regional UNESCO conference on braille uniformity for southern Asia took place in 1950. This led to a conference with global scope the following year. The 1951 congress found many conflicting braille assignments:
The congress recognized the role of English contracted braille in establishing a partial international standard, and recommended that alphabets follow existing conventions as much as possible.
Common extended correspondences.
The following assignments include common secondary vowels and consonants: Whenever a second "a-" or "d-"based letter is needed in an alphabet, use of the same secondary braille letter is common. Additional alternative letters are used in some braille alphabets. English grade 2 braille correspondences are given below for recognition; these are often the basis of international usage.