If Onset is always a consonant.What is onset for the word "Obligate"?

edit

This word does not begin with a consonant, so it has no onset. However, vowel-initial words in English are sometimes (I don't know how often, though) pronounced with a glottal stop, so the glottal stop consonant would fill the onset position in 'obligate'.

Yes, an onset is a prosodic position that, at least in most cases in most languages, is filled by a consonant. (The only potential exception I can think of involves diphthongs, but even there the onset can be analyzed as containing a glide consonant.)

The onset is a unit within a syllable, not a word. So it is not the case that a word like "obligate" has an onset. Rather, each of the syllables making up that word can potentially have an onset.

"Obligate" has three syllables:

• [ɑb]
• [lɪ]
• [ɡeɪt]

The second and third syllables have the consonants [l] and [g], respectively, in onset position. I believe what's confusing you is the fact that the first syllable doesn't have an onset. Not all syllables do. For example, the words axe, ill, up, end, and oar (all one-syllable words) do not have onsets.

I hope this clears up your confusion!

There are three possibilities with regards to the onset:

1. it is empty (the syllable starts with a vowel rather than a consonant)
2. it consists of just one consonant sound
3. it consists of multiple consonant sounds, commonly referred to as a 'cluster'

Some languages do not allow empty onsets, whereas others do not allow clustered onsets. English is an example of a language that allows all three types.

Examples of onsets consisting of multiple consonant sounds:

• 'speak' [spi:k], where the onset is [sp].
• 'strap' [stræp], where the onset is [str]