It easy to explain transposing wood wind instruments but ambiguities occur when brass instruments are concerned.
This post attempts to clarify brass instruments.
Reminders: As far as transposing instrument is concerned, the eye (l’ oeil) and ear (l’oreille) doesn’t perceive the same thing .
For example a trumpet player reads then plays « C » but audience hears B flat.
The aim of transposing instruments is to keep the same fingering with different sounding instruments.
For example with saxophone.
In both cases written notes and fingerings are the same but sounds differently
Range of woodwind instruments is clearly established in the lower register.
There are few exceptions
-One key added on bari sax that can reach A instead on Bb on other saxophones.
-one missing key on English horn which cannot play the Bb of the oboe.
For any instrument, range in upper register is subject to variations due to player ability.
This simplicity is due to the wood wind instrument sophistication through holes and keys which allow to select partials (or harmonics).
Difficulty of brass instrument paradoxically results from their simple construction
Brass are, historically speaking, a simple flared tube with an embouchure which allow the emission of a note ,the fundamental or pedal tone , and its partials or harmonics.Those added sounds are simply, if I may say, selected by the way the performer blows in the tube.

Sound 1 Fundamental or Pedal sound depends on the length of the tube.
A mere sight of the series reveals a first group of problems.
1-Lower notes progress by skip up to the seventh sound
2- The following conjunct notes are not necessary diatonic to the first sound.
3- Some diatonic sound are out of tune in our equal temperament system.
4-The series doesn’t provide all chromatic sounds.
Our modern instruments are
-The association of several tubes (trumpet, saxhorn) to supply the missing notes or the nearest notes of the equal temperament system.
This is done by mean of valves or slide that broaden the length of the initial tube
-or by fusion of two instruments, as seen in the french horn (F+Bb) or F-attached tenor trombone . The switching is made with a key.
Three valves provide seven combinations corresponding to seven bugles covering a tritone chomatically.

Therefore, a Bb trumpet is made of a (no valve) Bb trumpet, an A trumpet, an Ab trumpet.. and so on up to E (the lowest)
Those seven combinations are similar to the 7 positions of the slide of the trombone.
A crook is another possibility to lengthen the tube . We can,for example, turn a Bb trumpet into A trumpet but with many defects.
A fourth valve extends the tritone range to an octave (12 combinations) which ease the reach of pedal tones especially on saxhorns.

However, the fourth valve doesn’t necessary mean a larger range. It may only provide a better intonation but also bring new problems which possibly require more valves to solve the problem.
A second group of problems is due to the impossibility to cover the overall series with a single instrument.
Only instrument with large diameter can reach pedal tones.
Overcoming the harmonics 7 require a small diameter instrument
The Ratio diameter/length of the tube is also important to reach the pedal tones.
The shape of embouchure is another factor to select partials which determine the timbre. It may help to reach some pedal tone but too weak in strength to be efficient.
For instance a bass saxhorn (tuba ) can reach the contrabass ‘ lower notes but they are weaker, so the larger range of the bass is at the expense of the strength of lower notes.

However this instrument is only effective in the following range
It must not be confounded with the Bb low bass Tuba which uses a French horn embouchure. Its range is
sounding a second below.
That’s why we can hardly express the range of brass instruments. Furthermore,instrument makers have evolved lot of devices to enhance the playing which broaden the range and panel of instruments.
A third group of problems comes under instruments making and nomenclature.
The first difficulty is
-The name of instrument which doesn’t reflect the register. For example, the saxhorn Bass has the range of contrabass Eb but lower notes haven’t their strength.
-A given name correspond to different instruments according to countries For example » bombardons » mean either contrabass or include tenor bass. In an other hand the word tuba refer either to a non-homogeneous group of low brasses or a specific name for bass-saxhorn (in C or Bb) or Bb saxhorn contrabass.
–Confusion of register and instrumental group. Especially US Baritone (saxhorn) and euphonium Bb which is not a saxhorn; its timbre is different .
The complete range of Jupiter euphoniums should stops the confusion.
Another difficulty is to muddle tonality of the original tube and the concert tone.
For example, the trombone is a Bb instrument because in first position ( the shortest tube) it emits a Bb and its harmonics but it’s a C instrument (meaning non transposing) because the trombonist reads and hears Bb and so does the audience.
Caution:Modern tubas follow this logic contrary to former saxhorn- tubas that are transposing instruments.
At least some puzzling writing conventions
-No accidentals on the score for old instruments (Naturals)
-Treble clef for « tuben » according to the saxophones logic to keep a common fingering for different (lower Major sixth (Eb instrument ) or lower Major ninth(Bb instrument) Therefore transposing instruments). This practice is found in the salvation army brass band where all part but bass trombone part are written in treble clef
But the same part may be written in bass key sounding a higher minor third (Eb instrument ) or a lower major second(Bb) instrument
or -written in bass clef with concert tone ( non transposing instruments).
The problem often occurs with euphonium. The reason is more or less historical when trumpet players, only reading treble clef and playing three valve instrument, switched to euphonium especially with 4 valves.
Fingering of trumpet Bb is similar to the one of euphonium sounding a ninth lower.(An Eb euphonium would sound a sixth below)

(Figures refer to valves)
Another historical reason is the octaves determination according to Organ pipes length.
Sound threshold tallies with a 64 feet pipe
-32 feet pipe corresponds with lower C of a 5 strings double bass
-16 feet correspond with lower C of cello
-8 feet correspond with lower C of viola
Since Tenor voice was written an octave above its sound,(baritone saxhorn) is written in treble clef but sounds as a 16 feet pipe (an octave lower) On the other hand a bass saxhorn is written (as a bass) in bass clef ; Being a 16 feet pipe its sound register is similar to the baritone’s one.
Double bass is written like a bass but as a 32 feet pipe sounds an octave lower .
To summarize
Baritone-Bass____ Different Clef —Same sound
Bass -Double bass—Different sound—-Same clef
« Real » tuba player read bass clef and play concert pitch with a specific fingering which is different according to the tuba tonality). In open, trumpet plays G (treble clef) while tuba plays F'(bass clef). the 1-3 valve combination yields D to the trumpet but C to the tuba that can possibly replace the 1-3 by 4
Treble clef writing presents an ambiguous zone due to the large compass of euphonium

In the above example , are the framed notes written in concert pitch for a tuba player or in transposed notes written for a trumpet player?
So I advise to use the tenor clef to write those notes in concert pitch.

Euphonium is Bb according to the trombone logic explained above and therefore non transposing in bass clef
Nowadays tuba scores are generally written in concert pitch (Non transposing)
Since tuba range is very broad, professional tuba players, king of the transposition choice the best tuba for the given tonality .

For the composer the difficulty concerns writing for wind orchestra especially for amateur,unaware of transposition .
From the trumpet range.

The three valves saxhorn baritone is written an octave lower
F contrabass is written a fifth below the baritone and the Eb contrabass a sixth below the baritone but start on G
Bb bass and contrabass are written an octave below the baritone but start on G with a smaller span (2 octaves).
Tenor trombone shares baritone range while ‘euphonium and bass trombone add a tritone to the lower register

French horn covers the range of the trumpet and of the baritone
However the whole range should theoretically be divided up into low horn (II-IV) and high horn (I-III)

Actually , the standard writing is as follow

Below is the advisable range (étendue) of brass instruments (in concert tone)

Avoid pedal tones in writing for trumpet and tenor trombone . Pedal tone should be assigned to bass trombone
French horn can emit pedal tones only through the Bb tube but only two pedal tones (B-A) are reliable.
When writing for a specific instrument
-it’s advisable to consult the player.
-Whatever the skill of the player one must never forget the general melodic rules: remain in the median register with a progressive reach by step of extreme notes .
More about transposition
Tables of brass instruments (notation and concert pitch equivalence
