How come when you drop a third a half step, it’s called a minor? But when you drop a fifth a half step it’s called diminished? 
Things that keep me awake
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Way back in the beginning in the Music Theory stage, I learned some things that don’t make sense are because of the “Dead Monk Rule.”
I think I smell flowers and hear slow music!
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The good news is you could call it a diminished third and it would not be wrong. Just not usual
There is a standard way of naming intervals. It is all about the distance from the tonic, which is your reference point. Try to focus at how far you are from the tonic (as in the number of frets) instead of looking at the half-steps you raise or drop. Some intervals have more than one name, they sound the same, just different context.
It is important that your first try to understand how the scales are constructed.
Say, C major is C-D-E-F-G-A-B-C.
If you look at the intervals of the major scale, this means
(1)Tonic C + (2)full step (D) + (3)Full Step (E) + (4)Half Step (F) + (5)Full Step (G) + (6)Full Step (A) + (7)Full Step (B) + (8)Half Step C
THe intervals are always standard, they tell you the distance from the root note. Say you are in C Major
C - Tonic
C# - Minor Second (1 half step above C major)
D - Second or Major Second (2 half steps above C major)
D# - Minor Third or diminished third (3 half steps above C major)
E - Major Third or “diminished fourth” (not usual, but it can be called that) (4 half steps above C major)
F - Perfect Fourth (5 half steps above C major)
F# - Augmented Fourth or diminshed fifth (6 half steps above C major)
G - Perfect Fifth (7 half steps above C major)
G# - Augmented Fifth or minor 6th (8 half steps above C major)
A - Major Sixth (9 half steps above C major)
A# - Minor Seventh or augmented 6th (10 half steps above C major)
B - Major Seventh (11 half steps above C major)
C - Octave (12 half steps above C major)
So why some intervals have 2 names? Because if you are playing a major scale, the intervals are as above. When you play a minor scale, the intervals change, and therefore the way you name intervals change too. C minor scale is
C (root)+ D (full Step)+ Eb (half-step)+ F (Full step) +G (Full step) +Ab (Half step) +Bb (Full step)+C (half step)
Eb is the same as D# right? We cant call it D# in this case though. THat is because when you play diatonically, you can’t name intervals the same note, say D-D#. Since in a C minor scale we already have D, the next note up must be an E. It can be flattened or augmented, but it must be called E regardless
So, when you play F# over a major C chord, you’d call it augmented fourth if you are also playing the G which is the fifth. you can’t call it “diminished 5th” because that would mean having 2 fifths in the same chord, which breaks the diatonic rule of single intervals. Even though the notes are the same and they would sound the same, these are just the laws created to standardize notation
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There is only 12 notes distance from C to C. You can move forward or backwards on the fretboard and try to get used to the distance and how they sound when you play the root note and whatever interval you choose after it.
For example, you are playing a chord CMajor which is C-E-G (C is the root note, E is Major Third , G is Perfect Fifth )
ANd the band asks you to play C minor. This means you drop the major third a half step. This would apply to any major chord situation. So, from C, the Minor Third is D# (3 half steps above C). You play C-D#-E (Which is now C minor chord)
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The reason that some intervals become minor when flatted and some become diminished when flatted has to do with what they were originally. Perfect intervals (like the 4th, 5th and octave) remain perfect when inverted. Major and minor intervals reverse their tonal quality when inverted (ie. a major 3rd becomes a minor 6th when inverted, likewise a major 7th becomes a minor 2nd inverted). Major intervals become minor when inverted and minor intervals become major when inverted. Perfect intervals remain perfect when inverted (that is why they are called perfect) . A perfect interval will never be minor or major. However, if you flat a perfect interval, you make it’s size smaller or diminished. If you sharp a perfect interval then it becomes larger or augmented.
Finally, a major interval when made smaller by one 1/2 step becomes minor. So C to E is a major 3rd, C to Eb is a minor 3rd and C to Ebb (enharmonically D) is a diminished 3rd. And to be clear, major intervals inverted become minor, augmented intervals inverted become diminished but perfect intervals always remain perfect ; hence the name.
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Special thanks to both Mike and Frank for taking the time to respond in a detailed fashion. I will read these repeatedly, and gain a little bit each time I read them.
I mean it. I really appreciate you both taking the time. It’s great info.
Jack
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By the way Mike, I’m glad you survived the “Frozen Tundra” of northern PA> 
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I’m not going till October 11th. We should be okay though; we are bringing half the house.
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Reading and thinking about $35 Picks keeps me awake 
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Since I know nothing about music theory, Mike & Frank are making my head hurt, LOL! I’m gonna need to re-read those a few times for sure. Thanks for the detailed explanations guys.
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My wife played piano in her younger days, and she’s tried to teach some music theory to me while I hold my banjo. She says it’s easier to explain and understand if you have a keyboard in front of you.
Honestly, I don’t know why I married that woman! Can you imagine me holding a piano on my lap?
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Totally agree about the keyboard. I think I’m going to get a Roland FP-10. I’ve always wanted to mess around on the piano. It seems like it’s all kind of laid out there for you to visualize a little easier.
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I think she’s right about the keyboard, so that was probably a good reason. 
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Why do they call it a parking ramp when the only place you can’t park is the ramp?
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Ha! That sounds like a Steven Wright joke…!
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George Carlin…
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Why do they call them “Stop Lights”?
Sometimes they’re green and you can just go right through! Wouldn’t it then be a “Go Light”?
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I thought his was “Why do people drive on a parkway, and park in a driveway”?
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that too
they are both much older though…
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I could stay up all night if I tried to think about how all the notes, chords and licks I played through a 3 verse song made any sense. Normal conclusion: they didn’t😂
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