If the time signature is 8/8 or 4/4 and let's say we have 8 eighth notes in a bar the picking should just be down up down up down up, etc. But what happens if we have 7/8? Particularly in the next bar. After we play the first bar the 7th eighth note was played downwards so should the first note of the next bar be played upwards?
While, as said by the previous answers, such meters can normally be sudivided into little chunks, it is in my experience not a good idea to let this influence strumming patterns etc. to directly: this is prone to give exactly the experience that many people associate, dislikingly, with odd meters – a "jumpy" sound, as if something is just missing or artificially plunged into the rythm.
In my band (where we have lots of unusual time signatures), we tend to try avoiding the unevenness or spread it out across the bars and the intruments as much as possible.
Sometimes, the best thing is to just keep the "natural" pattern going as if it were a 4/4. The extreme application of this is the grooves Led Zeppelin preferred in odd meters: John Bonham would usually play extremely even, often actually staying with something that sounded like a very reduced rock rythm for most of the time. Listen to Black Dog. I quite like this approach; it preserves a powerful beat with no jarring jumps, while still giving some exciting quirkiness of the odd meter. But of course such an approach doesn't always work, it quite limits the ability to build in common syncopated accents etc..
Yet, for guitar, you can often keep to a simple alternating down/up pattern even when obeying an accent pattern of an odd meter. If a 7/8 is not to fast, the natural thing might be a down/up-pulse in 16th notes; that way you end up on a downstroke on each 1 without further ado. If that's too fast, simply reversing the direction on every other bar might be fine as well. Sure, you won't have the potentially more powerful downstrokes on the "classical" emphasised beats, but always accenting those beats with rythm guitar tends to sound a bit stupid anyway. It's often better to only "feel" the accents.
If you find none of this works satisfyingly and you need to build in some "switch", I'd normally try to place it somewhere in the middle, so the last stroke[s] are already coherent with the first ones of the next bar. For instance, in a 7/8 I might prefer
If you're not playing with a pick, there's another option that borrows from Flamenco strumming techniques: you can do e.g. two down strokes in a row, but both within one hand movement, using the fingers for the first stroke and the thumb for the second. I find this can give extremely smooth results and can be placed pretty much anywhere in the meter.
This is quite another issue to begin with. In traditional folky finger picking, the thumb has a similar "steady beat" role as the down/up movement in strumming, but usually slower. It's quite difficult to incorparate this into an odd meter; I'd probably try do actually keep it going, possibly over multiple bars, until it's eventually in sync again. But I don't have much experience with that.
For less rythmic picking, perhaps just single-note arpeggio, there shouldn't be much of a problem: which string you pluck doesn't follow a fixed underlying beat anyway, so you can pretty much try anything you feel fits in the meter. What's ideal depends again on the musical context.
Uneven time signatures such as those not considered to be "duple" or "triple" time are typically sub-divided into groups of two and three.
For example, 5/8 yields two common divisions = 2+3 and 3+2
Similarly, 7/8 yields 2+2+3, 2+3+2, 3+2+2, and 4+3 as common sub-divisions. Obviously, the measure can really be divided any way you wanted. So, 5+2 is also welcome as well, for example.
Your picking pattern should reinforce how the measure is divided. If you've got a 3+2+2 division, your picking pattern should follow that as it will help rhythmically emphasize what is happening musically. This is also known as rhythmic support.
Hope that helps.
In fact this isn't just an issue for "unusual" time signatures. It is an issue when there is an odd number of notes in a phrase, which can happen in a 4/4 song, and is not always the case in a 5/4 song.
Here is a common rhythm pattern in 4/4 with an odd number of notes:
I've notated it with strictly alternating stroke directions, and you can see that if you choose that stroke pattern, the first bar starts with a downstroke, while the second bar starts with an upstroke.
And here is a 5/4 pattern with an even number of notes:
You can see that in this case, strictly alternating stroke directions results in the same pattern for each bar.
But that doesn't answer the question - what do you do if you're alternate-picking and when you come to repeat the phrase, the phrase starts with an upstroke?
The answer is, whatever sounds best. If you need to keep alternating strictly, in order to maintain the flow, then that's what you have to do. If that spoils the way it sounds, then you need to find a place where you can get back to the "right" direction.
Being able to change between all-downstrokes, strict alternation, and a hybrid, is a skill you should learn.
Although it should normally be possible to do two sequential downstrokes without affecting your flow; you can still think of it as a "breath" in your phrasing if you like. For example, the 4/4 pattern shown above could be played as
For picking I'm reading strumming. In a normal 4/4 bar song, the strum will be 4 down , with up-strums in between if you want. In 5/4 or 7/4 the same will apply, with down-strums on the beats and up-strums on the 'ands'. As jjm says, emphasis on the first of each split will keep a good rhythm pattern going. There should be no need to start a new bar with an up-strum.
An example of what you envisage is in 12/8 time - 4/4 with triplets - where beats 2 and 4 will be strummed up-down-up, as opposed to 1 and 3, which go down-up-down. As in the Beatles 'All My Loving'.
If the piece is very quick, and you can't strum the beats just with downstrokes, then you will have to start each alternate bar with an up-strum, and this is tricky, because up-strums by definition will not sound as heavy (accented) as the expected downstrum.
8/8 time is quite unusual. Written as 4/4 more often than not.
Addendum - PLEASE don't fall into the trap a heck of a lot of self-taught guitarists make for themselves : playing an up-strum on the last 'and' of each bar, but not being able to change to the new chord quickly enough. Their solution is to up-strum the open strings, to allow fretting fingers time to form the new chord. In some keys it can be got away with; in others it just sounds awful. If one can't change fast enough - leave out the last 1/2 beat strum in each bar !!
Watch any good rhythm player, and notice how the strumming arm flows with a regular motion. The up/down movements are not jerky. With some of the above answers, the strum pattern, whatever it is, will result in jerks.
By playing the main beats all with downstrokes, the 'ands' are with upstrokes. This will keep a steady flow going. When a rhythm player wants to change the pattern in the middle of a song, it is then so easy - the pattern of the strumming arm doesn't change, only the number of occasions the strings are sounded.
For example, in a song with 4 strong beats per bar, all will be downstrums.Changing the feel to say, reggae, with the same tempo, just miss the strings on all the downstrums, but flick them on the way up each time - reggae. Strum arm pattern - exactly the same.( I'm trying to keep it simple).
What I'm advocating is that in most rhythm patterns, including odd time sigs., is that EACH BAR needs the same motion with the strumming arm.Thus, subtle pattern changes are easy to do, and will sound smooth. A lot of the above suggestions will, I feel, cause the rhythm to sound jumpy - unless, of course, that's what is wanted or required for the song. Not usual, though.