The term ‘geminate’ when applied to consonants refers to at least two distinct phonetic realities.
Firstly, for continuants, i.e. fricatives and sonorants, a geminate will simply be a longer form of the corresponding simple consonant. Thus the geminate nasal in [inˈnato] innato ‘innate’ is essentially an articulation of /n/ that has a longer duration than the corresponding non-geminate sound [n].
In the case of stops, however, matters are slightly more complex, as these sounds by definition are instantaneous and hence cannot be prolongued. In this case, then, a geminate must be envisaged as two discrete articulations of the consonant in question, with the first being unreleased, i.e. having no audible release burst.
To envisage what is meant by an unreleased stop, consider English words like like act, capped or dogged, in which one stop immediately follows another. Thus in act, /t/ follows /k/; in capped, /t/ follows /p/; and in dogged, /d/ follows /g/. In careful speech, both consonants in each sequence could in principle be fully pronounced, each with its in own release burst: [akt], [kapt] and [dɒgd]. However, in relaxed speech the first stop in each pair would typically be unreleased, with the speaker moving their tongue or lips into the correct articulatory position for that consonant but delaying release until the following consonant is articulated, a state of affairs which can be indicated using the ‘combining left angle above’ diacritic, as in [ak̚t], [kap̚t] and [dɒg̚d].
Note that in each of [ak̚t], [kap̚t] and [dɒg̚d], although the first stop has no audible release, it still has a residual auditory signature, due to the fact that the speaker has moved the relevant articulators into position. Thus [ak̚t] act, for example, is in principle distinguishable from [ap̚t] apt despite the pre-consonantal /k/ and /p/ being pronounced as unreleased stops.
In the examples just considered, the two stops in each sequence are non-identical, which allows the speaker to choose between fully articulating each member of the pair or, more commonly in speech, pronouncing the first member as an unreleased stop. However, when the two immediately adjacent stops are homorganic (i.e. pronounced at the same place of articulartion) the first is necessarily unreleased. In other words, the first articulation in a geminate stop is necessarily an unreleased one. Thus Caribbean Spanish pronunciations such as [ˈpugga] for purga or pulga are more exactly transcribed as [ˈpug̚ga]. However, the ‘combining left angle above’ diacritic is usually omitted when transcribing geminate stops, as the unreleased articulation of the first member of the pair is entirely predictable.