Q: In the compound चरणकमल (lotus feet), how come does the word चरण come first in the compound?
The compound is explained by the sūtra 2.1.56 उपमितं व्याघ्रादिभिः सामान्याप्रयोगे। ~ समासः तत्पुरुषः.
Here is the meaning of the sūtra.
A word which is compared (उपमितम्) by words such as tiger (व्याघ्रादिभिः ) is compounded with that word when the common feature is not told (सामान्याप्रयोगे ).
In the case of चरणकमल, चरण (foot) is compared to कमल (lotus), and the common feature because of which the comparison is made, such as purity, is not mentioned (सामान्याप्रयोगे).
सामान्य means that which is common.
अप्रयोग means non-employment
In the sūtra 2.1.56 उपमितम् 1/3 व्याघ्रादिभिः 3/3 सामान्याप्रयोगे 7/1 ।,
the word which is compared is taught in the 1st case.
The meaning of the word taught in the 1st case in a sūtra enjoining compound is termed उपसर्जन by the sūtra1.2.43 प्रथमानिर्दिष्टं समास उपसर्जनम् ।.
By this, the word चरण gains the term उपसर्जन.
Then, the sūtra2.2.30 उपसर्जनं पूर्वम् । gives पूर्वनिपात, placing of a word in front, in the context of compound, to a word termed उपसर्जन.
This is why चरण comes first in the compound चरणकमल.