Dagesh

The dagesh is a diacritic that is used in the Hebrew alphabet. It takes the form of a dot placed inside a consonant.
A dagesh can either indicate a "hard" plosive version of the consonant or that the consonant is geminated.
In Modern Hebrew, the dagesh only changes the pronunciation of ב‎ bet, כ‎ kaf, and פ‎ pe: in those letters, it turns a fricative sound into a plosive sound, regardless of the dagesh's grammatical/orthographic origin, kal or ḥazaq; gemination itself is generally not pronounced in any consonant.
The dagesh was added to Hebrew orthography at the same time as the Masoretic system of niqqud.
Two other diacritics with different functions, the mappiq and the shuruq dot, are visually identical to the dagesh but are only used with vowel letters.
The dagesh and mappiq symbols are usually omitted when writing without niqqud, however, dagesh may be added to words that would be ambiguous without niqqud. The use or omission of such marks is usually consistent throughout any given context.

Dagesh qal

A dagesh kal or dagesh qal may be placed inside the consonants bet, gimel, dalet, kaf, pe and tav. They each have two sounds, the original hard plosive sound, and a soft fricative version produced as such for speech efficiency because of the position in which the mouth is left immediately after a vowel has been produced.
Although there is debate, some scholars suggest that prior to the Babylonian captivity, the soft sounds of these letters did not exist in Hebrew, but they were later differentiated in Hebrew writing as a result of the Aramaic-influenced pronunciation of Hebrew. The Aramaic languages, including Jewish versions of Aramaic, have these same allophonic pronunciations of the letters.
The letters take on their hard sounds when they have no vowel sound before them, and take their soft sounds when a vowel immediately precedes them. In Biblical Hebrew this was the case within a word and also across word boundaries, though in Modern Hebrew there are no longer across word boundaries, since the soft and hard sounds are no longer allophones of each other, but regarded as distinct phonemes.
When vowel diacritics are used, the hard sounds are indicated by a central dot called dagesh, while the soft sounds lack the mark. In Modern Hebrew, however, the dagesh only changes the pronunciation of bet, kaf, and pe. Traditional Ashkenazic pronunciation also varies the pronunciation of tav, as does Yemenite pronunciation. Some traditional Middle Eastern pronunciations carry alternate forms for dalet.
In Ashkenazi pronunciation, tav without a dagesh is pronounced, while in other traditions it is assumed to have been pronounced at the time niqqud was introduced. In Modern Hebrew, it is always pronounced.
The letters gimel and dalet may also contain a dagesh kal. This indicates an allophonic variation of the phonemes and, a variation which no longer exists in modern Hebrew pronunciation. The variations are believed to have been: pronounced as, as, as, and as. The Hebrew spoken by the Jews of Yemen still preserves unique phonemes for these letters with and without a dagesh.

Pronunciation

Among Modern Hebrew speakers, the pronunciation of some of the above letters has become the same as others:

Letter	Pronounced like	Letter
vet	like	vav
khaf	like	chet
kaf	like	qof
tav	like	tet

Dagesh hazaq

Dagesh ḥazak or dagesh ḥazaq may be placed in almost any letter, indicating a gemination of that consonant in the pronunciation of pre-modern Hebrew.
This gemination is not adhered to in modern Hebrew and is only used in careful pronunciation, such as the reading of scripture in a synagogue service, recitation of biblical or traditional texts or on ceremonial occasions, and only by very precise readers. However, the rules of the dagesh ḥazak still influence pronunciation in modern Hebrew, though not by gemination: in all cases where Biblical Hebrew geminates a letter, modern Hebrew retains a dagesh inside the letter, which influences pronunciation in the following way: in ב‎ bet, כ‎ kaf, and פ‎ pe, it turns a fricative sound into a plosive sound ; in all other letters, it is not pronounced.
The following letters, the gutturals, almost never have a dagesh: aleph, he , heth , ayin, and resh. A few instances of resh with dagesh are recorded in the Masoretic Text, as well as a few cases of aleph with dagesh, such as in Leviticus 23:17.
The presence of a dagesh ḥazak or consonant-doubling in a word may be entirely morphological, or, as is often the case, is a lengthening to compensate for a deleted consonant. A dagesh ḥazak may be placed in letters for one of the following reasons:

The letter follows the definite article, the word "the". For example, שָׁמָיִם in Genesis 1:8 is הַשָּׁמַיִם in Genesis 1:1. This is because the definite article was originally a stand-alone particle הַל, but at an early stage in ancient Hebrew it contracted into a prefix הַ, and the loss of the 'l' was compensated for by doubling the following letter. In this situation where the following letter is a guttural, the vowel in 'ha-' becomes long to compensate for the inability to double the next letter - otherwise, this vowel is almost always short. This also happens in words taking the prefix לַ la-, since it is a prefix created by the contraction of לְ le- and הַ ha-. Occasionally, the letter following a he which is used to indicate a question may also receive a dagesh, e.g. Numbers 13:20 הַשְּׁמֵנָה הִוא.
The letter follows the prefix mi- where this prefix is an abbreviation for the word min, meaning 'from'. This is because, similarly to the case of the definite article above, the loss of the 'n' at the end of the abbreviated min is compensated for by doubling the following letter. For example, the phrase "from your hand", if spelled as two words, would be מִן יָדֶךָ. In Genesis 4:11 however, it occurs as one word: מִיָּדֶךָ miyyadekha. This prefix mostly replaces the usage of the particle min in modern Hebrew.
The letter follows the prefix she- in modern Hebrew, which is a prefixed contraction of the relative pronoun asher : the prefix's first letter is dropped and its last letter combines with the following word's first letter, which is therefore doubled and a dagesh ḥazak appears in it. For instance, "who descend" becomes where the dot inside indicates the letter has been doubled. This prefix is far less common than the full asher in Biblical texts, but mostly replaces it in Modern Hebrew.
It marks the doubling of a letter that is caused by a weak letter losing its vowel. In these situations, the weak letter disappears, and the following letter is doubled to compensate for it. For example, compare Exodus 6:7 לָקַחְתִּי with Numbers 23:28, where the first letter of the root has been elided: וַיִּקַּח. Lamed only behaves as a weak letter in this particular root word.
If the letter follows a vav-consecutive imperfect, which, in Biblical Hebrew, switches a verb between perfect and imperfect. For example, compare Judges 7:4 יֵלֵךְ with Deuteronomy 31:1 וַיֵּלֶךְ. A possible reason for this doubling is that the וַ prefix could be the remains of an auxiliary verb הָוַיַ being contracted into a prefix, losing the initial ha, and the final ya syllable disappearing and doubling the next letter.
In some of the binyan verbal stems, where the pi'el, pu'al and hitpa'el stems themselves cause doubling in the second root letter of a verb. For example:
* Exodus 15:9 אֲחַלֵּק, pi'el-stem, first person singular future tense
* in the phrase הַלְּלוּ יָהּ, where hallelu is in the pi'el-stem, masculine plural imperative form
* Genesis 47:31 וַיִּתְחַזֵּק, hitpa'el-stem

Rafe

In Masoretic manuscripts the opposite of a dagesh would be indicated by a rafe, a small horizontal line on top of the letter. This is no longer found in Hebrew, but may still sometimes be seen in Yiddish and Ladino.

Unicode encodings

In computer typography there are two ways to use a dagesh with Hebrew text. The following examples give the Unicode and numeric character references:

Using combining characters:
*bet + dagesh: = U+05D1U+05BC or בּ
*kaf + dagesh: = U+05DBU+05BC or כּ
*pe + dagesh: = U+05E4U+05BC or פּ
Using precomposed characters:
*bet with dagesh: = U+FB31 or בּ
*kaf with dagesh: = U+FB3B or כּ
*pe with dagesh: = U+FB44 or פּ

Some fonts, character sets, encodings, and operating systems may support neither, one, or both methods.