The suffix of the comitative is ne. It is always preceded by the plural suffix i, since the comitative is always in plural form, independently of meaning. In a comitative form of a noun, the ne suffix is followed by a possessive suffix. For example, the comitative of vaimo (wife) is vaimoineni (with my wife), vaimoinesi, vaimoineen, vaimoinemme or vaimoinenne, depending on the person.
Adjective attributes in comitative have no possessive suffixes, e.g. uusi vaimo : uusine vaimoineen. The same applies to numerals, e.g. tuhannet järvet : tuhansine järvineen, and pronouns as attributes, e.g. (ne hyvät :) niine hyvineen.
The comitative indicates company and often corresponds to the English preposition “with” in the meaning “together with, in the company of”. The form is always in plural. For example, Hän saapui vaimoineen (He arrived with his wife) does not mean that the man has several wives. Similarly, Lähettäkää hakemus liitteineen osoitteeseen... (Send the application with its appendix(es) to...) does not specify whether there is one appendix or more.
The comitative is mostly used in sayings, such as kaikkineen (in all, totally), kimpsuineen ja kampsuineen (with all stuffs), niine hyvineen (without getting anything/more; literally: with those good things), and puutteineenkin (with its/his/her defects, too). The saying omin nokkineen (on his/her own, alone; literally: with own bill/beak) is incongruent: the attribute is in the instructive.
In standard literary language, comitative forms are used as adverbials that relate to a verb. They express that the action described by the verb is performed so that the subject acts together with someone or something. The comitative is rather rare and almost never used in speech, except in some special phrases. Instead of vaimoineen, one usually writes and says vaimonsa kanssa, using the posposition kanssa with the genitive. However, one might say Syön omenat kuorineen (I eat applies with their peels); here kuorien kanssa would be unnatural.
A noun in the comitative may be followed by päivineen, the comitative of päivä (day), which may strengthen the meaning to refer to “in all” or be used just for style. For example, the sentence Tehdas paloi koneineen päivineen (The factory burned, with its machines) emphasizes that the machines burned, not just the building, but it can also be read to infer that all machines burned.
Another strengthening addition is kaikkineen, e.g. koneineen kaikkineen. The addition is not an attribute, so it has a possessive suffix. Compare with the attribute construct kaikkine koneineen (with all its machines).
Nowadays comitative forms are used more freely especially in marketing and similar contexts, often in translated texts. In such usage, the comitative acts as a counterpart of the English preposition “with”, and often it does not really relate to a verb. For example, the heading text Saturnus: Kaasuplaneetta kauniine renkaineen (Saturn: A gas planet with beautiful rings) sounds somewhat odd and artificial. In literary style, one would rather write Saturnus: Kaasuplaneetta ja sen kauniit renkaat, though this presents the planet and the rings as two things, instead of presenting the rings as a subsidiary. The adessive case and to some extent the instructive case are also used in a similar manner. The Finnish language really lacks a natural general expression of the type “A with B”.
Style guides emphasize that the comitative indicates something that happens to appear in the company of something else. We can write Hän tuli lapsineen (He came with his child/children), because a person can logically come with or without children. But in actual usage, the comitative is used more wildly, in almost any context where we might use “with” in English, e.g. Rauma on kaunis kaupunki vanhoine taloineen (Rauma is a beautiful city with its old houses) and Eläin katsoi minua suurine silmineen (The animal looked at me with its large eyes)., which would more properly be written using the adessive or the instructive: suurilla silmillään or suurin silmin.