सन्धि-प्रकरणम् - Sandhi
The close proximity of letters is called सन्धिः
★ वर्णानाम् अतिशयितः सन्निधि संहिता ।
The word संहिता is a techincal word used by Panini Maharishi in his sutra.
The word सन्धि / संहिता mean the same thing.
When two letters are in close proximity, a significant change happens at the juncture as per the rules of sandhi. It is called सन्धि-कार्यम्. This change of letters at the juncture is called सन्धि-कार्यम् which is generally called सन्धिः
स्थानि - A place in which or with reference to which the change happens. THe place could be a letter or a syllable or more than one syllable.
आदेशः - That which is ordained in the place of स्थानि is called आदेशः. In other words, आदेशः is that which replaces स्थानि (or स्थानि is that which is replaced by आदेशः)
1. At the juncture (सन्धि), x can be replaced by a new letter z. xxx + yyy = xx[z+y]yy. Here, x is स्थानि. z is आदेशः
2. At the juncture (सन्धि), y can be replaced by a new letter z. xxx + yyy = xx[x+z]yy. Here, y is स्थानि. z is आदेशः
3. At the juncture (सन्धि), both [x+y] can be replaced by a new letter z. xx[x + y]yy = xx[z]yy. Here, [x+y] is स्थानि. z is आदेशः