This is a messy and time-consuming (but historic and important) technicality. Among the requirements for a Will to be legally valid, it must be witnessed by at least two persons and the witnesses must “subscribe their names to the Will“:
Unfortunately, a legally valid Will cannot enter into probate by itself! Specifically, the law (going back centuries to at least the feudal ages) requires that a Will be “proved.” This is, in fact, the fundamental and eponymous purpose of a Probate Court — after all, the word “probate” comes from the Latin verb “probare” — meaning “to prove” (our words “probable” and “probably” come from, and are similar words in English to, the Latin verb “probare”).
A Will is “proved” by the Court’s taking the testimony of the witnesses, showing that the Will:
Nevertheless, having the Testator’s witnesses appear and testify in court, just like having the Decedent’s lawyer “read” the Will to the family, has long since become obsolete. Instead, what is done is to have the witnesses not only “subscribe their names to the Will” but also, often in one and the same process, sign a sworn statement (technically, an Affidavit) before a Notary, stating what they have witnessed, making the Will what is known as “self proving.” (Legally, subscribing the Will and making the sworn statement are two independent events, with only the first being required to be done during the Will’s execution process, so making the sworn statement may be done afterwards — it is just much more convenient if the two events are done concurrently.)
Assuming that no objection is made, the sworn statement generally satisfies the Court’s requirement for “proving” the Will. Making this sworn statement is the reason for having a Notary present during or after the Will’s execution by the testator and subscription by the witnesses. Even the Notary’s presence has been obsolete for 25 years, as in 1981 RCW 9A.72.085 was signed into law, providing that, except for certain named exceptions (such as the making of Oaths of Office), a Declaration (also known as a “Certification”) under Penalty of Perjury is legally equivalent in Washington to a sworn statement. Consequently, since 1981 all that has been needed to make a Will “self-proving” is the witnesses’ signatures following the “magic language” of “I declare [or certify] under penalty of perjury according to the laws of the State of Washington (and so forth).” 2003 King County Probate Policy & Procedure manual, § 3.4.3.
If Decedent’s Will lacks the sworn statement or Declaration under Penalty of Perjury, you will need either to obtain it or to bring one or more of the witnesses to Court with you to testify. Assuming you would prefer the former, complete a Declaration of Witnesses to Decedent’s Will form.
and obtain as many of the witnesses’ signatures on it as you reasonably can. 2003 King County Probate Policy & Procedure Manual, § 3.4.3.
If you seek to obtain Declarations from multiple witnesses, it might be more convenient to send a separate Declaration to each of the witnesses and then to file all the completed, separate Declarations with the Court.