Glissando and Portamento

Glissando and Portamento both describe a playing technique that involve an audible transition (slide) from one note to the next note in a melodic line. There are several common misconceptions about these techniques and their notation as well as their execution in "real life" that need some clarification.

Glissando

Depending on which instrument you use them on you will in most cases not get a seamless transition but rather if you write a glissando from one note to the other note, you will get in a glissando the notes that lie in between. The classical example would be the Piano where you slide either over the white or the black or white and black keys from starting to end note and therefore either get a diatonic or pentatonic or chromatic glissando. But also with pretty much any other instrument, you will in most cases get all steps in between rather than a seamless glissando from one note to the next note. There are only a few instruments that are capable of creating a true glissando, which would be all string instruments, the trombones and depending on the player Clarinets (and Saxophones). (For reference here is the probably most famous clarinet Glissando from Gershwin's RHAPSODY IN BLUE:  https://www.youtube.com/watch?v=aEDq3ej7wjE )

However be aware that these instruments also can not play unlimited glissandi. The string length or slide length define what range is possible to gliss without needing to cheat by "hiding" a break in the gliss.

The standard notation for a gliss would be connecting start and target note either by a wavy or straight line. There hardly is any ambiguosity with these lines so the additional word "gliss." that many notation softwares offer as default is in most cases unnecessary clutter that transport redundant information which should be avoided in any sight reading circumstances.

A clear notation of the length of a glissando is rather tricky in convenvtional notation. Particularly on strings but also trombones if you don't write anything else to it, the general consensus is to start to glissando quite close before the target note holding the starting note for quite a while, particularly if the notation indicatates a quite long glissando.

One notation that has been established in the 20th century is to specify the length of the glissando by writing the desired length it should take as small note values in brackets above sometimes connected by a vertical line to the glissando line, sometimes not:

This makes it pretty clear when to start glissing and also when to rhythmically reach the target note. Showing rests between start and target note could create questions of whether to play or not. Sometimes you also see verbal instructions such as "start glissing on downbeat" or "slow glissando" which often should avoid that the players wait till the last moment to start to gliss (which often would be the default), however I personally prefer the above notation as it transports the information in a clearer way and is quicker to process.

Note that this is not necessary on timewise quite short glissandi that imply to start right away without leaving time to stay for long on the starting note.

The default of staying quite long on the starting note doesn't apply for example for harp glissandi which will always start right away, however also there in rhythmically ambiguous situations of where the start or end should land in the rhythmic grid, the above notation would be advisable as well.

If you prefer to leave more freedom on a passage like above regarding the rhythmic or even tonal placement you could only notate the start and very last target note, leave out the rhythmic guides in brackets and if needed even use headless notes and simply write "gliss. ad lib." on top which would mean "glissando ad libitum = glissando freely".

Portamento

A portamento (mostly used on strings) from a technical standpoint is the same as a glissando as it implies to slide from one note to the next note. The difference lies in the focus. With a glissando the focus lies on the actual sliding process while with a portamento the focus lies on the start and target note that get connected with a brief gesture of a slide. So the player would rather avoid to emphasize on the slide with a portamento as opposed to emphasizing the slide with a glissando. Portamentos in general are quite brief and should rather be considered as decorative effect rather than a specific playing technique. The notation is similar to glissando, sometimes with "port." written at the line.

A few additional and imporant remarks on these topics:

• For string players, playing with extensive portamento is rather considered bad taste, so the often seen and heard overuse of portamento samples used by inexperienced composers feels to them like putting too much icing on the cake. A cleverly used single portamento on a critical moment in the piece can create a wonderful effect, however having a whole melodic line being played with portamento feels like chewing gum and should rather be avoided.
• Don't mistake runs for glissandi. Particularly in woodwinds, I often see inexperienced orchestrators write glissandi while they actually mean runs. The scale material played between start and target note is often essential to imply the tonality and therefore you should write out the notes you wish to hear in between.
• Opposing to that, written out harp glissandi are rather uncommon and often clutter the score page. While it is also essential to indicate for the player which notes to use on the glissando, this is either done by only writing out the notes for the first octave and after that switch to the wavy or straight line or write out the pedal configuration for the gliss at the start of the gliss.