I've heard people talk about "high C" and "low C". What classifies a note as a "high" or "low" note? Also, what's above the high notes?
High and Low are relative quantifiers of pitch. Outside of context, these labels will mean nothing to musicians. That said, here are some circumstances in which "high (note name) and low (note name)" are used often:
- Scales or other phrases which are played at a range where an octave separates two notes.
- Certain instruments where the range is around two octaves will refer to a note as "high" or "low" to contrast it with the other note sharing its name. The same will be true for any two most extreme notes with the same name (think "highest E♭).
- Singers' ranges are often described with "high" and "low", because if you know the gender and/or general voice type, it's obvious which note is referred to. For classical sopranos, C6 is the "high C" often referred to. For basses, "Low C" would be usually C2.
- Referring to strings which differ by an octave; an example would be the guitar, having a low E string and a high E string (note that the fact that they're 2 octaves apart doesn't matter because there is no string at the E between them).
- Referring to instruments for which both re-entrant and standard tunings are common. Tunings for ukulele, "High G" (CEGA ascending in pitch) and "Low G" (GCEA ascending in pitch), are referred to this way.
In general, when it's obvious what note is being referred to in context, this naming convention can be used in place of always using scientific pitch classification or whatever other system.
If it wasn't painfully obvious, "high" is referring to the note which has a greater frequency. "Low" refers to one with a lower frequency.
If you are referring to the piano, then anything to the right of the middle c on the keyboard, is considered a "high" note (pitch); anything to the left of middle C, is considered a "low" note (pitch).
When people say "High C", they are usually referring to any C note to the right of middle C on the keyboard, when they say "Low C" they are usually referring to a C note that is below (or to the left) of middle C. Hope this helps!