The private letter is a non-literary, non-official, communication sent between family, friends, and acquaintances, see [Sarri 2018 : 27]. The subject matter is very broad [Sarri 2018 : 28] usually concerning family matters, e.g. [78426 III BCE, Arsinoite nome], the earliest private letter from Egypt, concerns the birth of a baby, or [31016 III CE] wedding congratulations. But quite often the line between a private letter and a business letter is not easily drawn and there can be much overlap, e.g. [43310 I BCE; 16266 86 CE].
The distinguishing feature of the private letter is that there are almost always warm greetings and good wishes. As these letters are usually written by private scribes (i.e. the letters are not the product of a scribal office), there is a wide range of ability - compare [28694 II CE] with its even lines, careful layout, and confident flourishes, with the less refined presentation of [25157 I-II CE]. But there is clear evidence of influence from the chancery of high Egyptian administration in some examples [28694 ; 30461 III CE, Antinoopolis; 31142 III-IV CE, Herakleopolite nome].
Often the scribe will write the letter and the author themselves will write the closing salutation e.g. [25289 I CE; 26586 II CE]; however, the son writing to his father from Alexandria likely wrote the whole letter himself [25933 100 CE, Oxyrhynchus]. The learned and witty author of  incorporated a Homeric allusion (line 10) in a letter to his son. Although he had it written by his scribe, he signed it himself and added a nickname (Ὀξυπώγων, ‘pointy-beard’) on the back.
Private letters make up a large portion of the corpus coming from every period and all geographic areas. The most recent work on ancient letters is [Sarri 2018] with a full bibliography on the subject [Sarri 2018 : 367-384], but see also [White 1986], and for an in depth investigation of letters concerning women in particular, see [Bagnall and Cribiore 2008]. This description will outline the features common to all private letters, and will comment briefly on the variations which are described separately.
[from name <nom.>][to name <dat.>][χαίρειν] [1116 III BCE], sometimes extended to πλεῖστα χαίρειν [9455 38 CE, Philadelphia; 27748 II CE, Arsinoite nome], or πολλὰ χαίρειν [28426 II-III CE, Tebtunis], or even more extensive greetings, e.g. πλεῖστα χαίρειν καὶ διὰ παντὸς ὑγιαίνειν .
Often there is an added [kinship expression] which in the Greco-Roman period may denote a family relationship e.g. τῷ πατρί ‘to (my) father’ [27499 II CE], τῷ ἀδελφῷ ‘to (my) brother’ , but the use of these kinship terms may not always be taken literally, see [Bagnall and Cribiore 2008 : loc. 253]. Additionally, an [epithet] may express different types of relationships, e.g. φίλτατος ‘most beloved’ [18827 41-54 CE;14010 117-138, Soknopaiou Nesos], and τιμιώτατος ‘most honoured’ [25449 I-II CE] are greetings seen more frequently among letters between colleagues in an official capacity, while γλυκυτάτος ‘most sweet’  expresses a more personal affection. Other letters have a very short introduction [30112 III CE, Karanis] or none at all [31355 III CE, Oxyrhynchus]. For a list of opening expressions see [Exler 1923 : 23-36]; for the relationship of these superlatives to similar Latin expressions see [Dickey 2004].
The opening address can sometimes be followed by a [statement] that prayers have been offered to the gods, e.g. [30991 III CE] τὸ προσκύνημά σου ποιῶ πρὸς τοῖς πατρῴοις θεοῖς ‘I offer prayers for you to our ancestral gods’, or to a specific god [30292 III CE, τῷ κυρίῳ Σαράπιδι], the latter god probably signifying a letter originating from Alexandria, see [Bagnall and Cribiore 2008 : loc. 261]. While the body of the letter can encompass a variety of subjects, often there is a polite [request] for a favour καλῶς οὖν ποιήσῃς, lit. ‘you will do well’ (i.e. ‘please’) [23907 150 CE, Oxyrhynchus; 29211 II CE].
The closing formula can be a simple [ἔρρωσο] [25287 I CE; 28701 II CE], [ἐρρῶσθαί σε εὔχομαι] [29265 II CE; 27748], and occasionally [εὐτύχει] [3461 152 BCE, Memphis] or [διευτύχει] [30581 III-IV CE]. Some examples go further to extend good wishes and health to everyone [31055 l. 24-26, III CE; 28695 l. 10-18, II-III CE, Arsinoite nome]. Other letters have none [30112; 31355]. Sometimes a simple [date@end] can be added after the closing salutation [28701; 27747] or occasionally a full regnal date .
The development of the format of letters is described in [Sarri 2018 : 87-113]. The early Demotic style format of very long narrow sheets is evidenced by examples such as [685 258-256 BCE, Alexandria] with a height of 31.5cm and width 10cm, the writing along horizontal fibres; see also [Sarri 2018 : 95-97]. But similarly long and narrow sheets can also be found in the Roman period e.g. [27748 II CE, Arsinoite nome] H. 31.5 x W. 12cm, and [31107 III CE] H. 33.5 x W. 10cm.
There is also much evidence in private letters for long narrow sheets cut along the height of the roll and turned 90 degrees so that the fibres run vertically and the writing therefore runs across the fibres, the transversa charta format, e.g. [5847 222 BCE, Elephantine] H. 9.5 x W. 33.5cm, and [5830 126 BCE, Herakleopolite nome] H. 17 x W. 32cm. The earliest surviving Greek private letter is transversa charta but, unusually for the Ptolemaic period, the fibres run horizontally [78426 270/232 BCE, Arsinoite nome] H. 7.4 x W. 26.1cm, see [Sarri 2018 : 91-95].
The pagina format, vertically oriented sheets similar to the Demotic format but with a width : height ratio of 0.5-0.8, is by far the favoured format of the Roman period [Sarri 2018 : 107-113, and Appendix II : 337-346]. Examples include [18827 41-54 CE] H. 32.6 x W. 16cm, [28795 II CE] H. 22.5 x W. 11.8cm, [30269 III CE] H. 25.6 x W. 16.1cm. Some examples of pagina format have the writing running against vertical fibres [27602 II-III CE, Philadelphia; 31055 III CE; 30581 III-IV CE]. There are also letters written on horizontally oriented sheets, along the fibres, found in the Roman period, e.g [31352 III CE, Oxyrhynchus] H. 7.7 x W. 17.5cm, [31142 III-IV CE, Ankyron Polis] H. 12 x W. 16cm.
Some letters are written on sheets with a squarish format, the writing along horizontal fibres [1116 III BCE; 25289 I CE; 28751 II CE]. Note [17628 100 CE, Alexandria] where the sheet is squarish (H. 16.8 x W. 16.7cm) but the writing is confined to the top third of the sheet. Sometimes the writing is against vertical fibres [2537 I BCE, Arsinoite nome; 30111 III CE].
Seals used to authenticate a private letter survive rarely, but an example can be seen attached to . On seals attached to letters see [Sarri 2018 : 140-143], and [Vandorpe 2019].
Most private letters are written as a single block of text, and while many scribes paid little attention to a formal layout [32206 III CE], some paid great attention to presentation. In such cases the opening address is often separated from the rest of the letter by having [χαίρειν] occupy a single line, positioned in the middle of the line [17288 68 CE; 25933 100 CE, Oxyrhynchus; 28795 II CE] or to the extreme right [26547 II CE; 28694 II CE]. The closing salutation is also often indented [17288; 40933 51/65 CE, Karanis]. Some of the private letters with a more careful layout, the use of ekthesis and eisthesis, also include a date at the end of their letters e.g. [17288; 25933; 26547; 40933]; this may point to a formally trained scribe familiar with producing official documents.
Some letters are made up of two columns [25933; 28695 II-III CE, Arsinoite nome]. Another papyrus carries two letters from the same person to two different addresses on the same single sheet [29211 II CE] neither of which were cut into separate letters; the relatively even bottom edge and lack of a bottom margin after the final greeting of [30955 III CE, Philadelphia] may suggest that it was cut from a larger sheet.
There can be a large lower margin , but some scribes clearly ran out of space as the writing became more cramped lower down the sheet, e.g. , and was carried on into the margins [31141 III CE; 31107 III CE] (left-hand margins), [78279 II Panopolis; 17627 100 CE] (right-hand margins).
Two variations of the private letter stand out as having a consistent structure: [letters of recommendation], and [letters of condolence].