Question: I am an amateur bass guitar player. At some point I decided to do something about my lack of theory knowledge.

Now I know a few basic scales and can read sheet music in simple keys (from zero to one to two flats/sharps). I understand in general how chords are built on degrees of the scale and know some basic ones. I understand what V/vii etc chord means. I know where notes on the fretboard are located.

But when it comes to real practice with a band, I'm completely lost. I can't seem to apply this knowledge in the process and reduce to catching roots by ear and playing cliches which often don't match the key and chord, and as I discover it I start alternating them nearly randomly (with the scale in mind if I'm happy enough to see it on the fretboard) until it sounds OK. 

**How do I bridge the gap between the theory and practice and start using the knowledge**?

Answer: 
Let's take a step back and let's look at what you've learned so far and where it falls in the grand scheme of musical knowledge. Theory and notation typically get intermingled as the same so identifying where each is helps understand:

* Scales (theory)
* Reading sheet music (notation)
* How chords are built (theory)
* Roman Numeral Analysis (theory)
* Notes on the fretboard (notation)

Notation is being able to read what someone writes down for music while theory is understanding it.

### Where the theory comes intertwines with practice:

Theory helps you understand a piece. My collage bass instructor would say every time he got a piece he would "make it make sense to him". The deeper you look the more patterns you'll see and seeing the patterns helps you plan out what you can play and relate it to other things you've played before.

One example is `C Dm G C` represents the same basic pattern as `E F#m B E`. Recognizing it lets you do more with less as both are I ii V I progressions that you can use similar if not the same line on.

If you know the piece is in C major and the progression is `I vi ii V`, it opens a lot of possibilities because you know the general harmony and direction. On option is you could reharmonize and utilize secondary dominants i.e. `I V/ii V/V V`. Another is you know the general set notes will be used and the direction the harmony will go. Another thing is you can take similar functioning chords and replace them. I, iii, and vi, ii and IV, and V and viio have similar functions in a key. They aren't identical function, but they will help change the flavor a bit.

Chromaticism is also another big tool especially if you understand more advanced substitutions like tritione substitution. For example the progression above can become `C Dm Db7 C` so you can spice up a progression like the one above.

If you know nothing about the piece then you'll have to relies on your ear and play on the fly. If your ear is strong enough you'll be able to figure out and utilize theory, but it's dependent on your ear. Likewise if you just look at the notation and don't look any deeper you'll have to play exactly what's written. If you look deeper, the theory will open up options for you.

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It's hard to see a lot of these patterns until you know how to look for them. I would suggest looking into what you're currently playing and common techniques first like the [ones listed here](https://music.stackexchange.com/a/26921/7222) and dissect them. Even just rehamonizing and playing the roots will open up a ton of new options.
The funny thing is, it's a lot easier to understand music theory than it is to understand how music theory will help your playing. It follows that spending time understanding the theory is time better spent than time trying to understand how it's helping.

When you're with the band, just put theory out of your mind completely. If you have to think about it, then you're not ready to use it anyway, and it just clogs up what should be an enjoyable an creative endeavour.

Remember that music theory comes from how music is practised, not the other way round. It's far more valuable to notice that C, E and G sound great over C major chords, and that adding a B to that sounds markedly different to adding a Bb, even though they both work (depending on the context). The fact that you're then using C Ionian or C Mixolydian (which differ by one note, namely B/Bb) is only a way to describe what your ears have worked out already.

Play when you're playing, study theory when you're studying, and trust that they will indeed converge and enrich each other.