6 Holes-Where Traversos and Whistles Meet

Pennywhistles and Baroque flutes.
Whistles are often used to introduce children to musical instruments based on the (mistaken) idea that all you have to do is blow. Traverso or Period flutes are often considered challenging for skilled musicians let alone beginners.
The current pennywhistle design was invented in the mid 1800s. Traverso players typically go back as far as the 1600s for their flute designs and avoid playing any style of flute after 1850 or so (when Boehm's redesign more or less created the modern concert flute).
In spite of these differences, these instruments are oddly entwined with each other. So much so, that I think learning one increases skill on the other and vice versa.

Let's start with an odd similarity in how these instruments are taught. They aren't. That's right, students rarely get any instruction on either instrument.
It is often assumed people can work out how to play a whistle by themselves because it has no keys and no special embouchure (mouth shape) is required. This means few people bother with much instruction. Now, it is true that if you ALREADY play a wind instrument, you may be able to work out the fingerings and get a decent start but kids being introduced to playing have a much tougher time. It strikes be as very odd and contradictory that previous musical experience is (somewhat unconsciously) assumed with an instrument that is often given to young children!
Meanwhile, traverso flutes have a reputation for being mysterious and difficult to learn partly because it has no keys (ironically enough) and don't play chromatic notes the same way modern flutes do. Finding a teacher is difficult not only because of how few concert flute players "dare" to pick up period instruments but also because there are so many different styles of flutes. If you do find a teacher, the odds are they aren't playing the exact same instrument as you and may not play flute at all (I ended up working with a Baroque bassoon player to get started). Books are still few and far between and generally assume the student is already a college-trained concert flutist, not a beginner. Generally, traverso players are left to muddle along on their own just like the whistlers.
The result is that both of these instruments are largely self taught because of the myth of their difficulty/easiness level and an assumption of previous musical experience.

Now the basic finger-hole pattern of these instruments.

Both are based on 6 finger holes, covered by the first three fingers of each hand with the left hand closer to the mouth. And both play a major scale using (basically) the same fingering pattern. The earliest traverso flutes from the Baroque era (1600s) have 6 finger holes plus one hole that is covered by a key that is almost always closed and therefore functions more or less like a 6 holed flute. Later traverso flutes had more keys added and with them more holes but the basic idea of 6 holes remained the framework design for most styles. (Fifes have the 6 finger pattern too in case anyone was wondering.)

Three Whistles-6 Finger Holes

Baroque Flute-6 Finger Holes Plus One Key

This means the fingerings of the pennywhistle often work on the traverso and vice versa though some fingerings need tweaking. This is especially helpful with the chromatic notes. Whistles have a slew of alternate fingerings for chromatic notes (so many it can be overwhelming) many of  which work on a period instrument. This can be wonderfully helpful since the fingerings for older flutes are not as detailed or extensive as the modern flute by a long stretch.

Modes, scales and folk music.
Both play the same basic set of scales or modes with relative ease. This means whistles and traversos can both play major/ionian, dorian, mixolydian and natural minor/aeolian and harmonic or melodic minor without too many half-holes or cross-fingerings.
Celtic music, old time fiddle tunes and a lot of other folk music use these modes and scales all the time. These styles use whistles pretty regularly and therefore, lots of this music will "fit" on the traverso flutes without resorting to chromatic fingerings. This means you can learn the ins and outs of the traverso on tunes that don't require the more difficult half-holes and cross-fingerings nearly as often as the more formal (and modulating) Baroque and Classical composers. And learning to play a couple of Bach or Telemann tunes on the whistle can make learning the half-holes and cross-fingerings a bit less challenging. Not to mention a great deal of Classical music actually was inspired by folk music so you may find some interesting musical relationships hiding in the staffs.

An interesting cross-over from the Celtic folk music realm; Because the Irish flute is (more or less) a traverso, an instruction book for Irish flute can be used with some traverso flutes. These books tend to have more absolute beginner information and you get some modal Celtic tunes in some of them.

Finally, there is one other advantage to learning the whistle with the traverso. It gives you a way to change your arm position and lower your arms from time to time. This helps with the repetitive use injuries musicians are so very prone to. Just the act of switching instruments lets you rest and recover without having to stop playing.

The more instruments I learn, the more each one informs my understanding and skill on the others. They open up worlds of musical styles and ways of thinking. They expand the possibilities and styles of music I experience and, with a little luck, let me share something fun and lovely with others.