Gnome rules, used in Kicad:

8.3.2. Capitalization

Two styles of capitalization are used in GNOME user interface elements:

Header capitalization
    Capitalize all words in the element, with the following exceptions:
        Articles: a, an, the.
        Conjunctions: and, but, for, not, so, yet ...
        Prepositions of three or fewer letters: at, for, by, in, to ...

Sentence capitalization
    Capitalize the first letter of the first word, and any other words normally capitalized in sentences,
    such as application names.

The following table indicates the capitalization style to use for each type of user interface element.
Table 8-3 Capitalization Style Guidelines for User Interface Elements

Element 	                            Style

Check box labels 	                    Sentence
Command button labels 	                Header
Column heading labels 	                Header
Desktop background object labels 	    Header
Dialog messages 	                    Sentence
Drop-down combination box labels 	    Sentence
Drop-down list box labels 	            Sentence
Field labels 	                        Sentence
Filenames 	                            Sentence
Graphic equivalent text:
for example, Alt text on web pages 	    Sentence
Group box or frame labels 	            Header
Items in drop-down combination boxes,
drop-down list boxes, and list boxes 	Sentence
List box labels 	                    Sentence
Menu items 	                            Header
Menu items in applications 	            Header
Menu titles in applications 	        Header
Radio button labels 	                Sentence
Slider labels 	                        Sentence
Spin box labels 	                    Sentence
Tabbed section titles 	                Header
Text box labels 	                    Sentence
Titlebar labels 	                    Header
Toolbar button labels 	                Header
Tooltips 	                            Sentence
Webpage titles and navigational elements 	Header