Odd behavior with LIKE?

I've noticed that if I don't preceed an underscore character ( _ )
with a double backslash ( \\ ), then a select using LIKE
ignores the underscore. For example, I have a couple of indexes
that end with "_ts" and a few tables that end in "ts":

From within psql:

select relname from pg_class;

pg_type
pg_attribute
.
.
.
raw_ts
accts
adjacents
areacodes
.
.
.
and so on.

Now if I change the select to omit all tables/indexes
that begin with "pg_" or end with "_pkey":

select relname from pg_class where relname not like 'pg_%' and relname not like '%_pkey';

the "pg_" and "_pkey" files are correctly ommitted.
However, if I also want to omit all tables/indexes
ending with "_ts":

select relname from pg_class where relname not like 'pg_%' and relname not like '%_pkey' and relname not like '_ts';

then not only is "raw_ts" omitted (which is what I want),
but so are the tables "accts" and "adjacents". I can
correct this with:

select relname from pg_class where relname not like 'pg_%' and relname not like '%_pkey' and relname not like '\\_ts';

and probably whould do so for all conditions:

select relname from pg_class where relname not like 'pg\\_%' and relname not like '%\\_pkey' and relname not like '\\_ts';

Question is: Why must the underscore character
be prefixed with a double-backslash?

Tim